Reasonable, I don't think you should make conclusions because of DMC4. Remember that DMC4 (PS3 version) got 1/3 of its sales from Japan, and that won't be the case on GTA IV.
I expect it to be close call in Europe, could go either way, but pretty equal. In US, the 360 version will dominate, just because the install base of 360 is 2,5 times as big as PS3's.
Then you have to consider the effect of price difference between the two. Some people will surely buy a new console just for GTA, and there the price may get it to 360's favour.
At this point the install bases (US and Others):
Xbox 360: 17,17 M
PS3: 9,07 M
When you look at that, it is very unlikely that the PS3 version would sell equal with 360 version ww, like some people think. It will propably go like 2:1 or at least 1,5:1.
I don't bother listing my last-gen games (PS2, Xbox, GC) to my collection cause there's too many of them. ;)
