For the record, I feel like a complete twat having both versions of the game and alternating between the two to try and compare things.
I'm not as wonderful a nitpicker as so many of you, but at first glance, both games are really quite on par.
First initial impressions are that the 360 image is definitely brighter and consequently has a slightly washed out look by comparison, but conversely, the PS3's image is a little too dark in dark scenes. This is kind of par for the course with a lot of multiplatform games I've found, especially GTA4.
I could swear the pop issues are a little worse on the 360, but I mean, I might have just been racing the horse too fast at those moments when I noticed things, or whatever.
The audio seems somehow richer on the PS3 version. It's like there's a bit more bass and power to every sound effect and the dialogue.
To any normal human being ( ) the framerates are fucking equal. I haven't stress tested it yet, but I shall shortly.
Really, they're both excellent. Are there are any specific things people want me to compare?
I have the 360 running at 1080i through component and the PS3 running at 720p through HDMI.