I guess it depends on what you want.

For the RPG fan, numbers-wise, you want a ps3. You'll have Suikoden VI and probably 7 and 8, Final Fantasy 13-16, Probably Disgea, .Hack, Breath Of Fire, Atilier Atlas, Kingdom Hearts, etc. The Xbox will have a few KOTOR and Jade Empire, Oblivion and probably the sequel(s).

For Shooters and GTA style games, Xbox wins. Halo, Half-Life, GTA. Probably for hack'n'slashes as well. Xbox has Ninja Gaiden, Assasin's Creed, etc. Ps3 probably gets Dynasty Warrior (though it might not be exclusive).

Puzzles: either is fine. Xbox already has Katamari and Spore, Viva Pinata. Ps3 has Lumines.

You know what kind of gamer you are. Rather than tell you which is better, I'll say it this way -- for certain gamers, ps3 is better, for others xbox is better.