I've gone back and played a few games on the 360 that I played on the PS3 and there's a little difference in some cases and next to none in others. COD:WaW, this felt much the same on the 360 and the PS3 although the PS3 version felt like veteran was much harder. Burnout Paradise, have to say the 360 version looks slightly better, but what it makes up in the graphical department it lacks in others. The online system on the PS3 is much better and less buggy. Dead Space was exactly the same on both platforms, but the 360 version had a lot more bugs (namely falling through the floor killing you).
The 2 that stick out for me so far have to be GTA4 and Midnight Club LA. Both of these looked and played like crap on the PS3, although I did stick with MC:LA till the end. I have a couple of screenies from both versions showing how vast the quality difference is really. The first one was taken whilst the car was moving so there is some motion blur, but not enough to cause a significant impact.
http://www.senture-mir.com/suvps3.jpeg
http://www.senture-mir.com/suv360.jpeg
I'm not trying to imply the 360 is better just because of a couple of games, just that there indeed is a difference in some games.