For the record, different pre-order bonuses from different retailers is really lame.
Amazon: Omega Boss Battle
Best Buy: Hardcover book that spans the gap between 1 and 2.
Gamestop: Alternate costume for Serah.
Amazon is the clear winner here.
Batman: Arkham City
Best Buy: Playable Robin
Gamestop: Joker carnival map
Amazon: $10 gift card and a print version of the Arkham city comic
Wal-Mart: $10 gift card
(note: A Batman Beyond skin is available, but not in the US. Rocksteady is a jerk.)