I'm going to say buy a 360, mostly because xbox fans are underrepresented here. There's a lot of great games on XBLA, some good older games should be fairly cheap by now, and you can make yourself a nifty Mii knockoff. There should be some decent bundle deals as well.
Demon's Souls Official Thread | Currently playing: Left 4 Dead 2, LittleBigPlanet 2, Magicka