I'm morally committed to never buying an Xbox of any sort, but if that doesn't matter to you (obviously not since you bought a 360) then I'm puzzled as to why one would want an FX over a 360. The 360's Live Arcade stuff is f*cking amazing. The FX...just sucks. Seriously, as cute as it is, its one of the worst consoles ever. There is hardly any selection of genuine "games" on it whatsoever, quality or otherwise. There is one semi-decent brawler, and one shooter. The rest is mostly RPGs, and action RPGs of a quality I would describe as mobile phone-grade. Its worse than a Supergrafx since at least a Supergrafx can play PCE games.
The only way a 360 is going to sit unplayed, and an FX would be actually played, is if you are just being contrary...or if you don't buy any live arcade stuff.