gamecube withouit a doubt. personally, i don't like x-box because of the games (although, i have to admit, Halo was a great game) and because of it's sheer size
EDIT
In reply to the post above mine... some games for gamecube ain't for kids. take resident evil for one. although, most of the games that i heard about for gamecube are kiddish but... other games like resident evil, just makes me want to get gamecube.