What do your friends have? If you have no friends, I would say get it for the 360. If you have a bunch of friends that have it for PS3, get it for PS3. And vice versa if you have a ton of friends with the 360 version.
I had to go with the PS3 version GTA will always be a playstation game to me. Besides i'm not worried about DLC. Guess the 360 will be collecting some dust for the next few weeks on a side note i picked up mariokart wii while i was out tonight. Still havent played it yet.