When comparing GTDNext vs Nozbe, the Slant community recommends GTDNext for most people. In the question“What are the best cross-platform task apps?” GTDNext is ranked 13th while Nozbe is ranked 35th. 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 Cross-platform
With Nozbe you can manage and get your tasks done anytime and anywhere. Nozbe has apps for iOS, Android, Windows Phone, OS X, Windows, and Linux, as well as a universal web interface. The sync works smoothly and handles off-line work responsively – all the work saved on your device will be synced to the servers as soon as you’re back online.
Pro Great real-time team collaboration features
Nozbe can be a perfect tool to manage communication, data, and workflow for teams – large and small. Features like comments, @mentions, admin/guest roles, attachments, and categories are great for managing all the documents and emails associated with a project. You can also invite people who don't have Nozbe account to participate in your projects.
Pro Nice and simple user interface
Nozbe is based on Getting Things Done (GTD), a productivity philosophy created by David Allen. However, you don't need to know the methodology to use the tool. You can just add projects, tasks, and attachments, and gradually learn to use the other features to organize your time even better.
Pro Practical integrations with some favorite tools
Nozbe integrates with email, Evernote, Dropbox, Google Drive, Box, Google Calendar, etc.
Pro Nozbe.HOW templates
Nozbe users curate a vast collection of free templates that share everything from favorite recipes to tried-and-true vacation itineraries.
Pro Highly intuitive and works the same way across all platforms
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 Limited free plan
After the free trial you can choose to remain on a free plan, but you’ll be restricted to a max of 5 projects and will have limitations on collaboration.
Con Task dependencies missing
Nozbe is great, but one shortcoming relates to "task dependencies" in projects. You should be able to sequence your tasks so that, when you complete a task, the next one that needs to be done becomes the "priority" task for that project. This is a vital feature that will make the program dramatically more useful. The company claims to be working on this improvement.
