When comparing Nirvana vs 24me, the Slant community recommends 24me for most people. In the question“What are the best cross-platform task apps?” 24me is ranked 48th while Nirvana is ranked 49th. The most important reason people chose 24me is:
The app has an Android version and an iOS version that syncs with the native reminders app.
Specs
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Pros
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.
Pro Integrates with Android and iOS
The app has an Android version and an iOS version that syncs with the native reminders app.
Pro Combines 4 apps into 1
The app lets you manage your notes, calendar, personal accounts, and to-do list from one place.

Pro Cuts down on procrastination
The self-proclaimed "assistant for everyone," 24me helps people break the cycle of procrastination and willpower exhaustion by automating things they may have otherwise put off until later.
Cons
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.
Con No desktop or browser client
