If the game gets a solid metascore of 85 or higher it will sell more because of the timed exclusivity...
More Xbots will buy it when it is released because of the timed exclusivity... And the PSNation will buy it as soon as it comes out for PS because it will have the reputation of being a great game.
If it scores below 85 on metacritic... Well, it won't sell well on either console.