I think that GTA IV's 360 version will sell more because 360 owners love achievements that up their overall gamer score. I'm still torn which system to get it for. I love my 360 but the fact that it used to be a Playstation exclusive has me swaying that way because of controls the controls on the PS3 will be virtually unchanged from previous versions.
I think to be considered a success on the 360 it has to outsell the PS3 version.