When comparing GTDNext vs Priority Matrix, the Slant community recommends GTDNext for most people. In the question“What are the best cross-platform task apps?” GTDNext is ranked 13th while Priority Matrix is ranked 47th. 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 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 Helps prioritize, rather than simply listing tasks
Priority Matrix helps me focus on what's truly important on my todo list, so I can work more efficiently.
Pro Super easy to share / delegate tasks
All I have to do is press a button, rather than drafting a whole email to one of my co-workers.
Pro Solid methodology
The software implements time proven methodologies; not just a to-do list.
Pro Lots of different views
Priority Matrix makes it really easy to see your task list in different ways: You can see it in the quadrant view, calendar view, a filterable list, and also a Gantt Chart if you have the business license.
Full disclosure: Originally posted by an Appfluence Intern (who was equally impressed with the software's layout and functionality before working there)
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 Expensive
Used to be free, is now $12+ a month.
