Actually I doubt advertising the hell out of it will affect much of anything.
I think existing 360/PS3 owners will go with their version. Unless the extra content is a serious amount of extra content only a few PS3 owners (and very big GTA fans) would buy a 360 just to get the extra stuff.
This is all about the fence sitters... but I don't think this will actually shift too many fence sitters one way or the other in the US. I mean how many people can there be who haven't bought a 360 already (with Halo 3, etc) out who will buy one because of GTA IV?
Likewise for PS3 - are there people in US who don't have a PS3 nor a 360 who would buy a PS3 on the basis of this game (given they can buy the more popular 360 console?)
I could of course be wrong but I expect it to be more like COD4 or AC- a very big seller in terms of units on both platforms, but only drive a modest spike in consoles for both (in the US).
Outside US is where the fun begins with regard to fence sitters. There is much more of a chance I believe of GTA driving actual console sales outside US I believe - but for whom is much harder to call IMO. PS3 seems to have more momentum at the moment, but 360 will have the extra content card. Of course it will likely be more associated with Plystation and PS3 does deliver BR, etc. which I believe has more impact on buyers outside the US right now.
Try to be reasonable... its easier than you think...
