Definitely Gamestop...I preorder it there last year...and I'm glad I did. Here is what you get for preordering it at the different places:
Target - A card that looks like a gift card except it is all black and has the KH 358/2 Days logo on it.
Amazon - A slip cover for the game...so basically a carboard box that says the game on it, that goes over the actualy package.
Gamestop - A poster of the game, and like Re: CoM, postcards of 358/2 Days.
Personally, I think Gamestop is the best, while Target is the worst....