When comparing GTDNext vs Teuxdeux, the Slant community recommends GTDNext for most people. In the question“What are the best cross-platform task apps?” GTDNext is ranked 13th while Teuxdeux is ranked 59th. 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 Fast, easy to use interface
On desktop, tasks are assigned to a single line. To see the full description, you can hover over that task. Editing can be done by double clicking, and re-ordering the events are as easy as dragging them around.
It also supports markdown - a common web formatting tool that allows italics by putting single asterisk () around words or phrases, and bold can be done by using two (*) before and after the phrase you want to be bold. It can also handle links by putting the text you want visible in square brackets, and the link in round brackets so this: [Slant Homepage](https://www.slant.co] would look like this: Slant Homepage
Pro Very focused, allowing it to be the best at one thing
Instead of trying to be a both a calendar and to-do app, it focuses much more on the to-do aspect. There is a calendar view, however your tasks aren't organized by time - just by order. You can reorder the day's events, and if you don't get them done they will automatically roll over into the next day, so you can be sure that you aren't ever missing anything.
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 Subscription based will cost money each month
Instead of a single time purchase, Teuxdeux costs $3 each month, or $24 for a full year. There is a free 30 day trial, so at least you can try before put any money forwards, but a one time fee - even if it's $10 - is cheaper in the long run than a few dollars each month.
Con No Android support
