some things to take in count :
will there be any technical difference between the 2 versions (frame rate drop?)
what will be the exclusive downloaded content : GTA4.05 ? or GTA4.5 ?
when will the exclusive come ?
what will be the entry price of the 2 consoles ?
==> but just considering the 2 userbases, it is obvious the Xbox360 will sell more
Time to Work !