MonoGame vs FlatRedBall

Support for iOS, Android, Mac OS X, Linux, Windows (both OpenGL and DirectX), Windows 8 Store, Windows Phone 8, PlayStation Mobile, PlayStation 4, Xbox One, Nintendo Switch, and the OUYA console with even more platforms on the way.

All the code is available to you ensuring you'll have the ability to make changes when you need to or even port to whole new platforms.

The framework implements the XNA 4 API, so games made in XNA can be ported to other platforms using this. This was the same API used by the Xbox Live Indie Games platform so there's lots of documentation online for it.

By leveraging C# and other .NET languages on Microsoft and Mono platforms you can write modern, fast, and reliable game code.

The community MonoGame has to offer is helpful and mature.

The performance on desktop platforms matches that of C++, but you still get all the pleasant features that C# has to offer.

Just check the commit frequency on github :)
https://github.com/vchelaru/FlatRedBall/commits/master

Plus Victor takes community input seriously and is known to shift around priorities based on the pressing needs of the community

Simplifies routine tasks such as adding entities and files to the game, or tuning parameters, via the FRB Editor called Glue

Very active chat on gitter: https://gitter.im/vchelaru/FlatRedBall
Victor (the creator of the engine) is available throughout the day to answer questions and solve any problems that may arise, along with the rest of the community members who are ready to assist in any way they can.

http://flatredball.com/documentation/
Very good documentation not only regarding the API details, but also lots of tutorials covering different aspects of using FRB, either on the code-side or on using any of its tools.