The entire Joystiq Network put up a poll asking it's readers on which platform they will be grabbing GTA on. The results are a little surprising to me, I would have expected that more people would have picked the 360 over the PS3.
In my eyes there is no advantage in the PS3 version, unless you count the "free" PS Network functionality. But I'll pay $50 a year to have a much better on-line experience. Not to mention the 360 version has exclusive DLC and achievements to boot.
Look for GTA IV to hit the streets on April, 29th.