A comparison for you to look at with everything broken down and videos and pics comparing the 2. IGN did have a review but no actual evidence (pics etc) so I tend to go with this.
GTA 4: Xbox 360 vs. PS3 - 360 Wins - VideoGamer.com
360 rules :D
Also the ps3 version is supposedly running at 630p not like the 360's 720p res.