The xbox is a decent chioce if you can't afford a pc, but in all honesty it has severe ram limitations that keep you from playing a lot of games. Also the guis to the emulators are rather basic and genrally speaking you can't display as much artwork as you can on a traditional fe, meaning it can be confusing to navigate.
The GUIs can definitely be considered limited compared to PC,but there are some pretty decent ones out there (especially for the emulators that support command line launching).
As for the ram constraints...
The number of games it can run are pretty vast, especially with virtual memory (No it is not as good or fast as having the actual ram there, but it hasn't really been detrimental to any of my experiences).
Also you have some great modern console games like Soul Calibur II, DOA 1, 2, & 3, House of the Dead III, Guilty Gear X2 #Reload, Lego Star Wars 1 & 2, and more. A lot of those you don't get on PC.
Of course on PC you have a LOT more control options, which can be a major advantage, especially if you like spinner/360° steering or trackball games.
The only real solution is to have one (at least) of each.
