All good questions. Sadly, I don't have any good answers.

Ease-of-use:
XNA-GS uses C# (simplified C++).
A7 uses Lite-C (simplified C).

Workflow:
A7 comes with tools for level, model, and scripting.
XNA-GS no asset creation tools, but you can code your own converters using XNA-Build.

Cost:
XNA-GS is free for the basic version.
A7 is free for the basic version.
The commercial versions of A7 are posted on our web-site ($59-$899).
The commercial versions of XNA-GS are not listed.

Conitec's Free Resources:
User Magazine || Docs and Tutorials || WIKI