According to IGN's recent review of GTA IV:
"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. 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."
So is IGN suggesting that people should opt buy the PS3 version? Not quite. The article continues:
"If you're wondering which version to buy, the best suggestion is to consider what your friends will purchase. That's because online multiplayer has made its way into GTA IV and it is awesome. And, like all great things, it's best played when drunk and with as many friends as possible."
So there you have it folks - buy the version your friends are purchasing. Makes sense to me.