PS3, which is supposedly the best gaming platform available, most processing power and therefore better graphics and fluid gameplay, plus doubles as a blueray DVD player which is useful if you have a good HDTV (no need to buy a separate expensive blueray DVD player).

Minor downside (which I don't personally care about) is Xbox version has the notion of various GTA4 "achievements" which you can accomplish besides all the missions and options side activities.