ImpactJS vs FlatRedBall

ImpactJS has an active user community that busily produces tremendous resources including videos, books, tutorials, walkthroughs and more. There are also comprehensive class documentation available.

Impact provides 2 types of collision detection; static and dynamic collisions. Both of which are easily integrated into game development.

Many plugins are available, including one called Impact++ which adds features like pathfinding and dynamic lighting.

Built-in map editor with support for tiling, collision layers, and actors.

Works with Box2d physics library, comes with all worthwhile elements for actors built in.

Extending the reach of a game developed with ImpactJS is easy due to the cross-platform nature the framework. There are Considerations around performance, resolution and audio, however all can be appropriately addressed when in development. Works in the browser via Canvas, even on mobile. Easily translatable into a packaged app.

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.