depends on what you like. Both are two totally different games.
IF you like shooters and intend to have friends over Timesplitters is a blast. The fun wears thin quickly though in single player mode.
SSX is a snow boarding game and in my opinion is your best bet for the money right now. IT is a good single or two-player game with a LOT of depth and gameplay.
Oh, if you are a graphics whore like me, both games look really nice...with the nod going to SSX