For those wanting to know which version looks better, the edge goes to the PS3. The textures and framerate are comparable, but the PS3 has far less pop-in.
While GTA IV is pushing the PS3 and 360 to the limit, it also runs amazingly well. Sure, there are framerate hitches here and there and (particularly on 360) there is some texture pop-in, but it actually runs better than I expected.
The 360 has richer colors, but the PS3 has better anti-aliasing making it look a little cleaner. Because GTA IV can preload onto the PS3 hard drive, the in-game loads are faster. Don't worry Xbox owners, the load times are rarely more than 30 seconds and don't occur very often. The slight visual edge goes to PS3, but the 360 is no slouch. Either version will do you proud.