I honestly don't think there'll be a difference. Maybe a slight frame-rate difference in certain areas but that's about it. The only reason I'll be getting it for 360 is because I have a 360. I bought a 360 ages ago because it was cheaper and I was wrapped up in the Halo franchise like a little heroin whore snatching at any fix they could get.
The idea of MP on GTA IV wasn't a dealbreaker for me. I've always played GTA for the single player experience, so the PSN vs XBL argument is irrelevant to me.