Assassin’s Creed 3 for the Playstation 3 will boast a whole extra hour of gameplay according to the games front cover. During E3 it was made quite clear that Ubisoft were uniting with Sony to offer exclusive additional content to the Playstation 3 version of the game.
I would assume that eventually the extra content will be made available for 360 owners too in the form of DLC. But there’s nothing to confirm this as of now.
Sony also has their own exclusive Assassin’s Creed game for the Vita dropping alongside Assassin’s Creed 3 in October.