About 70% of all multiplat games perform better on the 360, the rest is either a tie or run better on the PS3.
Although the difference is usually not that noticeable.
For comparisons go to LensofTruth.com
Assassin's Creed 2 runs a bit better on 360, Skate 3 is a tie
http://www.lensoftruth.com/?p=20016&page=2
http://www.lensoftruth.com/?p=16415
EDIT: Before I forget to mention it, the older the games the better they run on the 360 compared to the PS3.