I'm pretty sure Quaz51 is right about the PS3 version being lower resolution. But I also agree that with him when he says the PS3 version looks better. I really dislike the dithering the XBox360 version does. The PS3 version looks more natural and fluid. Although, I haven't seen the game in person yet. I'm basing my opinion purely on screenshots.
XBox360: http://dot50cal.the-horror.com/b3d/gta4/360/x3-image10.png
Look at the road and cars under the bridge (straight ahead) for the dithering.
PS3: http://img170.imageshack.us/img170/6858/wmplayer200804272348001sz1.png







