According to preliminary data released by Gamestop the Xbox 360 version of Grand Theft Auto IV has been outselling the PlayStation 3 version roughly 2-1 in North America.
Gamestop: GTA IV Selling 2-1 Better On Xbox 360 Versus PS3 : Read more
Gamestop: GTA IV Selling 2-1 Better On Xbox 360 Versus PS3 : Read more