I'm not in the USA, and have no actual experience of any of these STBs. But from what I read and hear, the Roku is the most capable and recommended internet STB. with Hulu, MLB, Netflix, Amazon On Demand, Comedy Central, etc. I believe popular video streaming services like Hulu, etc, are blocked and not available on Boxee.
The Apple TV is indeed just Apple iTunes, Netflix and nothing else.
Don't know anything about running SNES and GBA emulators on these things though.