When comparing Turtl vs Joplin, the Slant community recommends Joplin for most people. In the question“What are the best cross-platform task apps?” Joplin is ranked 5th while Turtl is ranked 15th.
Specs
Ranked in these QuestionsQuestion Ranking
Pros
Pro Multiple Platforms (Windows, Linux, macOS, Android, Firefox, Chrome)
Turtl has applications for all the major operating systems, as well as Android. In addition, there are extensions available for Firefox and Chrome that cooperate with the downloadable applications.
Pro Open source
The app is licensed under GPLv3 making it open source. This means that anyone can use the code and contribute. This also makes it easy to use on one's own server or for company solutions.
Pro Good security
After assigning a password to your account in Turtl, a key is created to encrypt the entire account. No data is stored on their servers meaning they have no access to unencrypted content. This is a huge leg up when comparing to other Evernote alternatives.
Pro Offline access
Pro Open source
Peace of mind that there is no malicious piece of software in the app.
Pro Encryption
Pro Cross-platform
Supports Windows, Mac, Linux, Android and iOS.
Pro Supports file attachments
Pro Supports markdown
Pro Supports tags for notes
Pro Web Clipper
Browser extension saves full pages, clean content, or screen selections to new notes.
Pro Optional CLI interface
Provides a CLI interface for terminal lovers.

Pro Supports multiple languages
English, French, and Spanish, Italian, German, Czech and many others.
Pro Active user forum with support from app creators
Support, troubleshooting, and new feature requests are easy to make at the user forum and you can contact the app creator directly.
Pro Dropbox support
Can sync using Dropbox.
Pro Nextcloud support
Pro Timed alarm reminder for to-do lists
Cons
Con No iOS app
While many other operating systems have a client, iOS does not have one yet (though it is planned).
Con No image embedding
Instead of image embeddings, there's a sort of poor man's substitute: image + description. If you add an image, you can create a description of any size and with all the formatting features. It can be used instead of image embedding but much more limited: only one image and only at the very top.
Con Internet dependent
Requires Internet connection to initiate offline mode, loses access to notes without Internet or server.
Con Sync issues with Android
If update on Linux then sync on Android, sometimes the Android sync will duplicate or remove some journal notes. Seems to be a bug in the Android app. The Linux app alone without sync with Android seems to work well using Dropbox. But don't expect changes you make on Android to sync properly back to Linux.
Con Sync issue under Linux
The Linux client has a bug that requires the user to click the mouse frequently in order for sync to proceed. This bug has been open for a while.
Con Gargantuan memory footprint
1GB+ of memory for taking notes.
Con Old school interface
Con Bloated
This project is suffering from feature creep and uses a lot of memory for a note-taking app.
Con App Image launches very slowly
On Linux, you can only install via App Image, which take 5+ seconds to launch. I can launch LibreOffice in under two seconds.
Con Sync issues on Windows
No way to change account settings and sync easily corrupted.
Con Weak tag searching
Joplin can currently only search for a single tag at a time. None of the boolean operators (AND, OR, NOT) are available for use in searches. This greatly diminishes the usefulness of tags in Joplin.
Con Can't create tags from the mobile app
