When comparing Taiga vs GTDNext, the Slant community recommends GTDNext for most people. In the question“What are the best cross-platform task apps?” GTDNext is ranked 13th while Taiga is ranked 30th. The most important reason people chose GTDNext is:
All the “Get Things Done” lists you would expect: next actions, someday, waiting, and scheduled.
Specs
Ranked in these QuestionsQuestion Ranking
Pros
Pro Supports both Kanban and Scrum modes
User stories can be organized in both Kanban and Scrum task management systems.
Pro Free and open source
Taiga is licensed under GPL with source code available on GitHub.
Pro Simple to use
Pro Comprehensive Agile software development toolset
Taiga tries offering a complete Agile software development toolset. It includes complete solutions for issue tracking, videoconferencing, documentation (in the form of a wiki) and either a backlog or a Kanban board for managing user stories.
Pro Built-in issue tracking
Taiga has built-in issue tracking tools. The issues can be organized by user-defined type, severity, priority, creation date, assignee, creator, tags as well as filtered by subject. Taiga can also integrate with GitHub, GitLab and BitBucket.
Pro Built-in wiki
Each project has a wiki. It has Markdown support as well as a WYSIWYG editor.
Pro Built-in video conferencing tools
Integrates with either AppearIn or Talky to provide a video conferencing solution.
Pro Migration from RedMine
Pro Export/Import feature
You can extract all your data from one Taiga instance and move it to another one. You can read more here.

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 No Kanban metrics
Taiga is said to support Kanban but it does not generate any of the usual Kanban metrics (cycle time, lead time) or graph (Cumulated Flow Diagram).
Con Can be overwhelming at first
Taiga presents users with a lot of information and functionality right from the beginning with little guidance. Figuring how stuff works might take a bit.
Con Too much functionality for small projects
While it's possible to disable any unwanted features (modules), the amount of functionality that's present might be more than a small, short-term project needs.
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
