When comparing Taskwarrior vs GTDNext, the Slant community recommends Taskwarrior for most people. In the question“What are the best cross-platform task apps?” Taskwarrior is ranked 2nd while GTDNext is ranked 13th. The most important reason people chose Taskwarrior is:
The command line interface puts powerful filtering expressions, context, annotations, tags, due dates, reoccurrence, and user-defined-attributes totally in your hands. It’s fast and low friction.
Specs
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Pros

Pro Command line interface
The command line interface puts powerful filtering expressions, context, annotations, tags, due dates, reoccurrence, and user-defined-attributes totally in your hands. It’s fast and low friction.
Pro Extensible
Taskwarrior has many front-ends, services, extensions, hook scripts, and capsules available. Taskwarrior keeps a list of contributed tools on their site.
Pro Lightweight and fast
Taskwarrior is written in the speedy C++ language.
Pro Tasks stored in plain text files locally
Future-proof and easily integrated with version control system. You never lose access to your data.

Pro Sync across devices
When used in conjunction with the Taskserver, it can sync tasks (conflict-free) across your Taskwarrior devices. This includes integration with Mirakel.

Pro Supported internationally
It has UTF8 support and is translated into many languages.
Pro Customizability
Taskworrior allows you to define custom attributes and reports to your needs.
Pro Self-hosted Taskserver
You can be in full control of your data.

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 Command line interface
A command line interface is a arguably overly verbose, consequently tedious, way to interact with a To Do list.
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
