Ugh! I'm having the same dilemma, actually. XBOX as well.
I've been looking into both for a while now. IGN rated Def Jam an 8.7 and I'm still waiting to see what they think about TW 2005.
However, I own TW 2003 (for the GameCube) and I love that game. Haven't played it in a few months but I'll go back to it as long as I live. Played the hell out of it for a good year or so. Now, two years later, 2005 looks to add more than enough to make me want to buy it and enjoy it even more.
As omega2K4 mentioned, have you seen the character creation mode? It looks absolutely incredible. You can even recreate your own swing to perfection.
I think I'm leaning towards TW, myself. Good luck.
By the way, if you're just looking for a good game in general, Burnout 3 is awesome.