When comparing Casual vs Momentum, the Slant community recommends Momentum for most people. In the question“What are the best cross-platform task apps?” Momentum is ranked 34th while Casual is ranked 67th.
Specs
Ranked in these QuestionsQuestion Ranking
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 Specifically designed for GTD
Pro Automated Next Steps
Next steps have a separate view. Tasks can be updated sequentially or parallel.
Pro Projects and Tasks can be endlessly nested
You can nest multiple projects with tasks, there is no limit.
Pro Has an offline mode
No permanent internet connection needed.
Pro Cross-platform
Web-based, progressive-web-app (PWA).
Pro Realtime synch
Data instantly updates on all devices.
Pro Clean design
Lots of space helps focus on the tasks.
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 Beta interface is confusing for first-timers
The interface is very opinionated (this is a good thing). But it is not obvious how to map user's own GTD variant onto the one the author has in mind. Maybe after it is in production, there will be better on-boarding.
Con No native apps
Con In Beta (development)
