When comparing GTDNext vs Doit.im, the Slant community recommends GTDNext for most people. In the question“What are the best cross-platform task apps?” GTDNext is ranked 13th while Doit.im is ranked 43rd. The most important reason people chose GTDNext is:
All the “Get Things Done” lists you would expect: next actions, someday, waiting, and scheduled.
Specs
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Pros

Pro GTD style lists
All the “Get Things Done” lists you would expect: next actions, someday, waiting, and scheduled.

Pro Automatic next actions
When it comes to choosing your next action, most to-do apps suffer from the same problem: All of your actions (sometimes hundreds in total) are visible to choose from. This can overwhelm users and cause them to give up on the system. GTDNext is different. In the "Next Action" view, only the very next action is shown for each of your projects. This results in a nicely consolidated view of only the things that need your immediate attention.

Pro Unlimited levels of projects and actions
In GTDNext you can set up an unlimited number of levels for your projects and actions, allowing you to plan your project and sub-projects in as much detail as needed.

Pro Areas of Focus
GTDNext supports the “Areas of Focus” methodology from Getting Things Done. You can filter all of your tasks to just show your work tasks, side project tasks, or personal tasks. There is also a global filter, so that when you’re hard at work you can focus only on your work projects and tasks.
Pro Option to visualize all projects and tasks in a single outline view
This is really useful because it prevents you from having to navigate through each project individually.

Pro Many keyboard shortcuts available
The number of keyboard shortcuts makes it easy to log or add tasks without removing your hands from the keys.
Pro Clean, simple layout
The apps borrows some design elements from Material Design-type design, which uses a slide-out menu. Everything is laid out in an easy-to-find manner.
Pro Daily review which allows for did-take VS should-take time comparison
A built-in daily and weekly review function that lets you compare the estimated time for tasks against the actual time they took, as well as productivity for the day/week.
Pro Low cost
Subscription value is generally lower than other platforms.
Pro Quick add from notifications
Pro Free apps for Android, iOS, Web.
Cons
Con No mobile app available
Con Web only
GTDNext does not have native apps for other devices, but it does work across PC/Mac/Linux on the web client.
Con No Google Tasks integration
Con Support non existing
Con iOS version no longer being updated
iPad upgrades stopped at iOS version 11, so if you want to keep Doit.im it is not possible to upgrade to iOS12. As there is no export facility, you are stuck.
Con Rarely updated
Con Offline sometimes
Con Nonexport and support desk say they will never include it (your data is theirs now)
Con Subscription based
While not expensive, it does cost money to get it for Mac or Windows (although you can view it in the browser for free). The subscription also adds automatic cloud sync, sync with Google calendar, adding tasks via email, manual sort, attaching Evernote into tasks, and more.
The subscription costs $2/month, or $20/year which is very reasonable
Con Sometimes it doesn't see your active subscription
Con Rip off
Con Not very customizable
There is no way for you to create your own "perspective".
