When comparing Google Keep vs Microsoft OneNote, the Slant community recommends Google Keep for most people. In the question“What is the best cross-platform to-do list app?” Google Keep is ranked 6th while Microsoft OneNote is ranked 25th. The most important reason people chose Google Keep is:
1-step to take a note on Android: tap 'take a note'. Web version: cursor is on the note itself.
Specs
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Pros
Pro Easy to use
1-step to take a note on Android: tap 'take a note'.
Web version: cursor is on the note itself.
Pro Super fast to use
The focus on a minimal interface makes everything fast to use on both web and mobile. This comes in handy for when a note needs to be jotted down quickly, as there is little to no load times or faffing about trying to get to a space where the note can finally be recorded.
Pro Unlimited and seamless sync across all Google connected devices, from mobile to desktop
When a mobile device isn't in use, Google Keep can be accessed as a Chrome app or as a website. Basically, no matter the device being used or the OS on it, there is always a way to access the app. There is also no limitation to how many devices this can be done with, freeing up the user to always have access.
Pro Works as a great bookmarking tool as links added can optionally include a preview image
Keep has a great feature that allows you to save sites you visit as a special note type with a link and a preview image.
Pro Automatic saving
Automatically saves new changes so that nothing is lost.
Pro Useful Android home screen widget
The Keep widget lets users quickly see existing notes and add new ones.
Pro Can share and collaboratively edit any item including lists
Keep allows sharing any item with other contacts and editing them together at the same time. This way a team can work together in real time, no matter their location.
Pro Notes can be grouped via labels
In addition to the ability to color-code notes, they can be labeled and navigated and grouped that way. This allows for easy organization, to then be found easily at a later date.
Pro Notes can be color-coded
Pro Really well implemented speech-to-text
Google has a lot of experience with speech-to-text functionality and they did a fantastic job with Keep. This means you can easily record spoken notes that can then be changed to a text based note.
Pro Notes can include photo and audio attachments
Notes can include multimedia attachments in addition to text.
Pro Supports drawing or writing notes by hand
Newer version of Google Keep now supports drawing on either a blank page, existing notes or in an image attached to the note.
Pro Both location- and time-based reminders
You can set reminders that bring certain notes to your attention when you arrive at a specified location or when it’s a specified time or date. Unfortunately you can only do one or the other for each note.
Pro Drag and drop sorting
You can move cards manually, which is great for prioritizing to-dos. Within a card, you can also drag and drop items.
Pro Archives finished tasks
Any finished task is archived and can be searched at a later date.
Pro Includes character recognition for text in images
Google Keep lets users take pictures of physical notes and makes the contents searchable within the app. This can be a convenient time saver for those that do not want to type out the necessary info, but rather take a quick snapshot of it.
Pro Cross platform
This app can be used seamlessly across all platforms including Android, iOS, and desktop computer platforms.
Pro Text can be added anywhere like a whiteboard
OneNote allows you to arrange text freely on the digital paper mimicking the way it would be done with a real pen on real paper.
Pro Comprehensive organization methods
Can have multiple 'tabs' at the top like in a web browser with many notes within each 'tab'.
Pro Stylus and digitizer pen support
OneNote can be used on all touch capable devices via stylus or digitizer pen making for an intuitive and easy way to take notes by hand, which can often be faster or just preferred over typing. It has the best pen input out of any note taking app.
Pro Sharing and collaboration
You can share your notes by inviting people with an e-mail, or by giving them a direct URL. You have control over what kind of access the user has on your note (read only, edit) and can revoke it at any time.
OneNote will alert you when a chance in made on your shared note.
Pro Integrates with other Microsoft services
Integrates well with Outlook, Office, and OneDrive.
Pro Powerful table editor
Adding and resizing rows and columns is easy and flexible. This makes for an easy way to adjust a note as it grows in content size.
Pro Supports hierarchical organization of information / notes
OneNote is designed to allow hierarchical organization of information. This is achieved with grouping into the following categories and UI elements (Notebooks, Sections, and Pages). Both Sections and Pages allow additional groupings that can be expanded / collapsed in list view.
This can be incredibly helpful for quickly taking notes and organizing them for work or personal projects.
For example, you could have a Notebook for "Cooking Recipes", with sections for "breakfast, dinner, desserts", where individual recipes would be page entries (i.e. the "Desserts" section would have pages for brownies, chocolate cheesecake etc).
Pro Strong search features let you quickly find what you're looking for
If you don't want to search through all of your notes, you can narrow it down by specifying the page, section, group, or notebook.
You're not limited to just searching through your text either. You can search for text in images, video recordings, and audio (this is off by default).
Pro All indented lines under the first can be expanded and hidden
Which makes it easy to quickly look at the high level of notes, and then drill deep into each point.
Pro Insert files into your notes
You can add a variety of different files into your notes including images, video, audio, and more.
Pro Lots of options for customizing fonts
You can fully customize your default font (the color, type of font, and font size). You can override this styles with the same options (and more) in individual notes.
Pro Very similar design for those used to Microsoft programs
If you have experience with other Microsoft programs such as Microsoft Office, you'll feel right at home with the design of OneNote thanks to the similarity of the menus.
Pro Article Web Clipper available on desktop and mobile
Has a web clipper extension on Chrome and Edge that can be used for clipping whole articles and images. Web clipping of articles is also available on Chrome mobile through the "Share" option.
Pro Cross-platform
Its available Windows Mobile, Windows Store, Android, a Desktop Version, and a Web app. Apple macOS and iOS are also supported.
Pro Offers flexible encryption
Unlike most other apps, you can selectively password protect, and fully encrypt, individual documents or entire folders. This is especially important to many people with data stored in the cloud. As of early this year, even the free version supports encryption. Microsoft has also enhanced their internal security methods for storing data on OneDrive (where OneNote performs sync), which makes it more secure than most of their competitors (including Evernote).
Pro Converts handwriting to text on Windows version
You can take handwritten notes on your mobile device with a stylus or digital pen and then convert the handwriting to text later on your Windows PC.
Pro On Android, you can tap a badge to start a note
The badge on Android hovers over on your screen similar to Facebook Messenger's Chat Heads. You can tap it and instantly start jotting down your thoughts without having to flip to the app first.
Pro Large variety of themes available
You have quite a few options for themes available with illustrated backgrounds. If you want something a bit more simple while still having personality, there are also several color themes.
Pro Add notes through email
You can choose what notebook the notes will save to. E-mail notes to your special Onenote e-mail and it will be accessible across all devices.
Pro Browser plugins allow easy saving of web pages
The ability to quickly file away web clippings is a key part of note taking.
Cons
Con Google can access and mine your data
You have to be okay with Google analyzing and using your data in order to use Keep.

Con No text formatting
No basic formatting like bold, italics. Makes it harder to do things like meeting notes. Keep is intended as a minimal and quick note taking app, but sadly for those that are looking for something more robust, the features are just not there.
Con Messy if you have lots of notes
Con Can be taken down some day
As Google is notorious for taking down services, you may end up with just a backup of your notes in XML format and a need to look for a substitute.

Con Not free/libre
This application is proprietary, and thus cannot be modified or freely distributed.
Con Google account required
You're required to have a Google account in order to use the app.
Con Organization is limited to just multiple tags for notes
There are no folders in Keep that you can move notes into. It makes the user interface more cluttered, and navigation gets more difficult.
Con No notifications when tasks are added to shared lists
Although the ability to collaborate on lists is advanced, Keep will not let you know when another person adds an item to a list you share.
Con No ability to undo changes
There is no way to revert changes or restore text you may have accidentally deleted (however, you can restore entire notes for up to 7 days after deletion).
Con Synchronization bugs
There are some issues with synchronizing data including laggyness and returning old/deleted items.
Con Not distributed
Cannot run on own servers as Keep will only run on Google's own servers. This increases the risks which are normally associated with handing over critical applications to a large (US) corporation.
Con No dedicated app for windows, linux, mac (except unofficial)
Con No integrated social media sharing
There is no integrated social media sharing if you directly want to post your note to Twitter, Facebook, etc.
Con Cannot share groups of notes (e.g., labels)
Google's equivalent of groups are labels, which can't be shared. By comparison, Trello allows you to share boards with others. Trello also supports grouping through teams. Google Keep has no such concept.
Con Does not integrate with Google Calendar or Tasks
It does not integrate with Google Calendar or other Google Apps with the exception of "Google Now." Keep reminders can appear on Google Calendar, but you can not get reminders and use Tasks at the same time.
Con Scrolling through notes is horrible
There is no options to decrease the surface area of the notes to make it easier to get an overview. Compare this to something like the Apple Notes or Evernote UI and you'll know what I mean.
Con Slow to save the updates
Saving is delayed by a moment and the saving status UI is unclear. You can lose updates if you close the tab too quickly.
Con Not good at all for longer notes
With almost no formatting options whatsoever it's hard to actually use Keep for long-form notes.

Con Too simplistic of an approach
Con Can be difficult to discover the features
Google Keep has slowly been introducing new features, but doesn't do a good job of helping you discover them.
Con Does not offer themes
There is no option for changing the color theme (such as a dark theme) unless you use a 3rd party extension.
Con There is a label limit up to 50
Con Delay between to-do entries
The keyboard closes and reloads between to-do list entries, so the first letters or words may be missed if you’re trying to quickly create a list.
Con No API
Con Notes get lost/deleted and can NOT be recovered
Since nothing is stored locally on the phone, everything can go missing. Google can't recover any of it.
Con No BlackBerry 10 app
No BlackBerry support apart from web.
Con No Windows Mobile app
No Windows Mobile app, and it is unlikely to ever be produced.


Con Not available for Linux
There is no native Linux client for OneNote available, and most likely will never be created. Users can use the web based client, but a native app would be a better choice for those that use the app frequently.
Con Requires a Microsoft account
You need to have a Microsoft email to sign up for OneNote (Outlook, Hotmail, or Windows Live).

Con Not open-source
This application is proprietary, and thus cannot be modified or freely distributed.
Con Text editing is weird (poor UX)
When clicking in the middle of a note, it adds something like a text-box positioned in the middle. If you move that field to the bottom right, it gets weirder. You can end-up with an empty note, having an empty text-box 1km to the bottom-right of it.
Con Syncing issues
It tells you there are conflicts between a note on the PC and on the Android because of the timing of the synchronization but doesn't try to resolve itself.

Con Sync isn't stable
Many users have reported synchronization issues. While not everyone will experience this, it can be difficult to troubleshoot, with some fixes resorting to editing registry keys in Windows when using the Windows client.
Con Platform dependent
The newer versions for Windows and Mac are converging, using the Windows style layout (with a more more consistent and usable UI).
The original OneNote for Mac for example is very different from OneNote for Windows. Because Microsoft has a vested interest in making sure the Windows version is superior, the Mac version tends to lag behind in terms of functionality. Tutorials and other forms of documentation available online generally apply only to the Windows version, which implies non-Windows versions are undocumented.
Even worse, because the documentation rarely, if ever, indicates it only applies to Windows, it's easy to waste hours trying to make non-existent features work. As such, the documentation is "negative documentation" (i.e. worse than no documentation at all).
Con No backup on Windows 10 free version
If you accidentally delete a section on the windows 10 free version of Onenote, there is no backup to fall back on.
Con Uninspired, bland user interface
Many of OneNote's competitors use Material Design and bold colors to create a minimal, clean looking interface.
OneNote has a bit more of an outdated look and can feel over crowded at times.
Con The added flexibility means your notes may feel cluttered
Since you can place elements anywhere you'd like on your note, you have to pay attention to how you structure it. For some this added flexibility is a huge selling point, but for others it could become distracting and makes notes feel difficult to digest.
Con You cannot use local OneNote files with the version of OneNote that is included with Windows
There is a difference between the OneNote that is included with Windows 10 and the OneNote that comes with Microsoft Office. The version included with Windows cannot use local OneNote files and requires you to be online to access your notes.
See here for this and other differences.
Con Search function is lacking advanced features
e.g. Searching for "friend" will find "friendship" and "friendly" (because "friend" is at the beginning of the word); using this query will not find words such as "girlfriend" or "boyfriend."
Con Menu/ribbon can feel crammed
OneNote offers a lot of formatting options, which results in a lot of options being crammed into the ribbon. It can feel somewhat cluttered and takes a bit of time to get used to.
Con Doesn't automatically sort (by name, size, date)
You can't choose to sort automatically. Just do it manually.