When comparing Omnifocus vs Nirvana, the Slant community recommends Nirvana for most people. In the question“What are the best cross-platform task apps?” Nirvana is ranked 49th while Omnifocus is ranked 63rd. The most important reason people chose Nirvana is:
Designed specifically for the Getting Things Done methodology as created by David Allen.
Specs
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Pros
Pro Defer date
Omnifocus allows users to setup the "defer date" which can be treated as "start date". Tasks will not be marked as available until that date, which should help to focus on the "available" task only.
Pro Forecast summary
The forecast summary shows a quick digest of the tasks you have for the day. You can select any day to quickly see what tasks you have planned that day. For extra functionality you can connect with the iPhone’s Calender app, which allows you to also see any other events you have planned that day.
Pro See an overview of all tasks via "Perspective"
The Perspective section allows you to see an overview of the tasks you have set. It also allows you to set contexts or see things you have flagged.
Pro Nearby and context-sensitive tasks
OmniFocus 2 allows you to set up contexts for tasks in certain areas. This makes sure you never miss a task when you're out running errands and happen to be near something you need to do.
Pro Streamlined for Getting Things Done
Designed specifically for the Getting Things Done methodology as created by David Allen.
Pro One of only few faithful implementation of GTD approach
The Nirvana approach is one of the few that successfully encapsulates the spirit of Getting Things Done.
Pro Flexible structure for simple to complex usages and workflows
Pro Clean layout
It has a very simple, understated design.
Pro Useful history of actions
Nirvana offers a chronological look at completed tasks that is good for reporting and improving future estimations. That said, it could be improved by incorporating a true journal view rather than a simple list broken up by week.
Cons
Con Mac and iOS only
Omnifocus doesn’t work on non-Apple platforms.
Con No native OS X & Windows apps
There are currently no native apps for Windows or OS X, although the company has claimed to be working on them.
Con The Pro service tier still does not have common features
Common features such as notifications and sharing between applications aren't available, even in the Pro-tier. Its only native integration with other applications is adding tasks via a specialized email address generated for your account.