When comparing WhatsApp vs Today, the Slant community recommends Today for most people. In the question“What are the best Material Design apps?” Today is ranked 3rd while WhatsApp is ranked 34th. The most important reason people chose Today is:
In addition to day, week and agenda views that have improved aesthetics and usability features compared to the stock app, there are also two views that combine agenda and month views in interesting ways. One approach shows a complete, color-coded month with agenda view beneath it and the second shows a complete month that pops up agenda view when clicked on a date.
Specs
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Pros
Pro Straightforward interface
The interface is very simple, fast and easy to use. You can start using it as soon as the app has been downloaded - it does not require setting anything up or registering. You can communicate with anyone in your contacts list that also uses WhatsApp. Includes useful features like seeing when a contact is currently available or typing.
Pro Widely used
WhatsApp reported 1.5 billion users in 2018. Your friends are most likely already using WhatsApp, so it's easy to get setup.
Pro Effortlessly builds your contacts by using your phone number as identification
WhatsApp will automatically find all of your contacts currently using Whatsapp and add them to your contacts. This makes it really easy to setup and given that it's the most popular messaging app it's likely that a lot of your contacts are already using it.
Pro Message mirroring on web app
WhatsApp can be used from the desktop via Chrome, Firefox and Opera browsers. Accessing it requires using the Android app to scan the QR code that's presented on web.whatsapp.com.
Pro Signal protocol
Whatsapp uses the state-of-the-art Signal protocol that ensure strong end-to-end encryption.
Pro One-on-one texts can be encrypted
WhatsApp has implemented end to end encryption for messages sent on it's service. Even WhatsApp has no way to read a users messages.
Pro Has improved common views and two great views that combine month and agenda views
In addition to day, week and agenda views that have improved aesthetics and usability features compared to the stock app, there are also two views that combine agenda and month views in interesting ways.
One approach shows a complete, color-coded month with agenda view beneath it and the second shows a complete month that pops up agenda view when clicked on a date.
Pro Polished agenda and month widgets
The app has minimalistic, semi-transparent, good-looking and customizable homescreen widgets for agenda and month views. Attributes that can be customized include choosing between light and dark themes, 12 and 24 hour clock, if the widget header should be displayed as well as adjusting opacity level and date range among other options.
Pro Actionable notifications
On-screen notifications have a shortcuts for map and ability to snooze them.
Pro Clean, good-looking design
Google Calendar has an easy to overview interface, that doesn't overwhelm with options. It's based on Google's stock Calendar application with modifications to user interface and functionality.
Pro Straightforward event creation
To quickly add an event, each view has an option to either click a plus button somewhere on the screen or long-press a date to bring up a form. The long-press form will allow entering only the event name and assume that it's an all day event, while the plus button form allows setting the event duration as well.
Both forms have an edit button for a adding more detailed information. This way in addition to selecting an event name and date, there's an ability to add location, start/end times, guests, description, color-code as well as choosing if it's an all day event, how often it repeats, how many and what kind of reminders are needed and setting your availability and privacy settings.
Pro Color-coding support
The app will color-code different calendars in different colors as well as allow selecting a different color for each event.
Pro Free 2-week trial
It has a 14-day trial and costs $2.99 to use full-time.
Pro Can set on-screen and e-mail reminder notifications
Google calendar allows setting how many reminders are needed with each having the ability to set how long before the event the notification should remind and if it should be done via on-screen or e-mail notification.
Pro Integrates with Google Maps for directions
The app leverages Google Maps when adding location for an event by offering suggestions as well as offers opening Maps to help get directions to an event via shortcut on a reminder notification.
Cons
Con Shares meta data with Facebook
WhatsApp shares your messaging meta data with Facebook if you do not opt-out in the settings. If you use the same phone number for WhatsApp and Facebook, the profiles can easily be linked.
See https://www.whatsapp.com/faq/en/general/26000016
Con Tied to carrier phone number
WhatsApp cannot be used by people without a smart phone with an active cell plan. If your phone is lost or damaged, or if your phone number changes, you cannot get messages sent by friends without them updating your phone number. You cannot send message to them until you are able to get SMS messages at your phone number.

Con Proprietary
Not open source.
Con Hard to migrate history cross platform
Migrating your chat history from Android to iOS or the other way around is not so simple. In some cases it requires a 3rd party app and in some the only way is to migrate specific chats one by one.
Con Web interface uses message mirroring
To use your computer to send messages your phone must be on with a consistent data connection.
Con Single device
You cannot use the same WhatsApp account on multiple devices (e.g. a tablet, a PC and a cellphone). The account is tied to the device with the specific cellphone number. Chat history is deleted when you sign in to a new account, unless you have created a backup.
Con No cloud sync
Messages are not stored on WhatsApp server for privacy and security reasons.
Con Limitation on shareable file sizes
WhatsApp now allows users to share files of any type, but there is still a 16 MB size limit.
Con No natural language input support
Con No weather forecast
