When comparing Casual 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 Casual is ranked 67th. 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 Tracks task dependencies using a graph
Pro Great to-do templates
Every to-do list & project can be saved as an amazing template that you can re-use in the future.
Pro Offers a high-level overview of projects
The workflow with dependencies shows where the things are at a glance. No more frustrations of how your project is going and who is doing what.
Pro Sync deadlines to Google Calendar
To-do items can be integrated with your day-to-day schedule using Google Calendar.
Pro Visual way of organizing your work
The workflow includes dependencies and shows you where things are at a glance.
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 Price
At its cheapest level, Casual is $9 per month for two users and two GB of storage. The trial period is short and there is no free version.
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.
