When comparing Nirvana vs Turtl, the Slant community recommends Turtl for most people. In the question“What are the best cross-platform task apps?” Turtl is ranked 15th while Nirvana is ranked 49th. The most important reason people chose Turtl is:
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.
Specs
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Pros
Pro Streamlined for Getting Things Done
Designed specifically for the Getting Things Done methodology as created by David Allen.
Pro One of only few faithful implementation of GTD approach
The Nirvana approach is one of the few that successfully encapsulates the spirit of Getting Things Done.
Pro Flexible structure for simple to complex usages and workflows
Pro Clean layout
It has a very simple, understated design.
Pro Useful history of actions
Nirvana offers a chronological look at completed tasks that is good for reporting and improving future estimations. That said, it could be improved by incorporating a true journal view rather than a simple list broken up by week.
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.
Cons
Con No native OS X & Windows apps
There are currently no native apps for Windows or OS X, although the company has claimed to be working on them.
Con The Pro service tier still does not have common features
Common features such as notifications and sharing between applications aren't available, even in the Pro-tier. Its only native integration with other applications is adding tasks via a specialized email address generated for your account.
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.