When comparing GTDNext vs Moo.do, the Slant community recommends Moo.do for most people. In the question“What are the best cross-platform task apps?” Moo.do is ranked 11th while GTDNext is ranked 13th. The most important reason people chose Moo.do is:
You can use tags like @next, @today, @tomorrow, and you will see all your tasks in agenda panel.
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 Agenda panel lets you see all your tasks
You can use tags like @next, @today, @tomorrow, and you will see all your tasks in agenda panel.
Pro Offline editor
Pro Very easy to get started with
Despite having many different features, Moo.do is extremely intuitive to use right from the start.
Pro Indefinitely nested lists of items
There is no limit to how many nested lists you can create. This gives you a lot of freedom structuring your ideas.
Pro Actively being developed
Pro Lets you view multiple panes
The multi-pane feature allows you to view lots of information on the same screen so there is no need to frequently switch between pages.
Pro Very fast to use
Very fast application which is useful when you want to brainstorm. No lags at all.
Pro Kanban
Kanban boards.
Pro Markdown
Markdown support.
Pro Multiple priority levels
You can quickly tag any of your content with one of the priority levels to help you stay organized and address the most important tasks first.
Pro Three different themes to choose from
You have three options (classic, outliner, and dark) for themes. The differences are specific to how your content is formatted (for example: how bullets are displayed, if at all). You can also toggle the "grayscale" option if you don't want your tags to be in color.
Pro Well designed interface
The design of the interface was well thought out. Despite the abundance of features, the interface is intuitive to navigate. It has a very clean, minimal design.
Pro Easily filter data using tags
You can #tag words anywhere throughout your notes in order to easily find them and keep similar notes organized.
Pro Turn your email into a todo list
Moo.do has an application for gmail that lets you fully organize your emails and turn them into a todo list. E-mails can be organized into different folders including what needs a reply, the priority levels, what is completed etc.
Pro Ability to zoom in/focus on a branch
Pro No limit on list items in the free version
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 Interface can feel distracting
Moo.do offers a variety of features that can make for a distracted experience. Rather than being encouraged to focus on the task at hand, you may find yourself tempted to flip through other panes.
Con Undo / redo
Undo / redo can be buggy.
Con No API
Con No wiki-style cross references
Con Inability to add images to the outline
This is now possible
Con Currently lacks notifications for changes in panes
