When comparing Emacs Org-mode vs GTDNext, the Slant community recommends Emacs Org-mode for most people. In the question“What are the best cross-platform task apps?” Emacs Org-mode is ranked 3rd while GTDNext is ranked 13th. The most important reason people chose Emacs Org-mode is:
This app's flexibility is based on its minimalist approach, giving the user near-infinite freedom.
Specs
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Pros

Pro Ultimate flexibility
This app's flexibility is based on its minimalist approach, giving the user near-infinite freedom.
Pro Absolutely free
Emacs with Org-mode is free as in beer and free as in speech – that is, it costs nothing and it’s totally open source.

Pro Files are usable anywhere at anytime
Users are not tied to one service provider, program, platform, or database engine.

Pro Incredibly extensible
There are many plug-ins for Org-mode, including Org-habits and Org-notify. If Org-mode lacks some piece of functionality, it is very easy to add it.
Pro Agenda views
Pro Excellent unofficial Android app (orgzly)

Pro Offline support
Pro Efficient features for deadline organization
Pro Supports plaintext spreadsheets
Pro There are a lot of extensions, for exporting to html, bootstrap, js-reveal and much more
Pro Quickly add rich text

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.
Cons
Con Unintuitive user interface
The key combinations are unintuitive and difficult to remember. This is probably because there are a lot of hidden "modes" depending on where the cursor is. Actions aren't paired with reversing actions like in other todo apps. For example, hitting shift-tab does NOT reverse the effect of hitting tab.


Con Android app isn't very good
There are several user-created apps for Android, but none seem to offer the same level of functionality as other to-do apps.
Con Not really cross platform
Although it is possible to get a lot of it working, no all in one, sync included, out of the box solution is available for mobile devices.
Con By default, a hard-to-read display
The default way of writing an outline or checklist creates a very messy wall of text that's difficult to read with no vertical spacing. You can manually add vertical spacing, but the Org operations don't preserve it. There are pretty-display modes, but you need to remember how to enable them, etc. etc.
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
