It depends a lot on what kind of games you like. If you like the regular nintendo-esque games like mario, wario, luigi, zelda etc then the gamecube is the obvious choice. The xbox however, has better hardware and a lot of solid titles too. Generally the xbox has more mature games, but it's really just a matter of preference. Personally I can't afford either, but until nintendo releases zelda and some of the other major titles I'd get an xbox instead.
__________________
|