Well, I don't think you should buy one yet, as there are very few great exclusive games to be played in Xbox 360, like Mass Effect, Halo 3, Halo: Reach, and Splinter Cell: Conviction. Wait until there is more Xbox 360 exclusive games, as currently, PS3 has the most great exclusive games compared other platforms.
If I am not mistaken, you want to play Splinter Cell: Chaos Theory, right? Why don't you wait for Splinter Cell Collection for PS3 on March? That would be much better than getting the Xbox version and just to play it on Xbox 360.