You most likely know how I feel about platform specific DLC and bloody pre-order bonuses and the like (I hate it, for the record) so I won't dwell on that. Instead I'll simply let you know precisely what PS3 owners are being offered over and above the traditional offering and be done with it! (Grumble grumble).
In short, folks who pick up the PS3 version over the Xbox 360 version will have access to four exclusive bonus missions for no extra cost. These missions cover the treachery of Benedict Arnold, amongst other things.
Sony also appears to be selling the cross platform connectivity between the PS3 and the PS Vita, providing additional content to those who purchase both Assassin's Creed III and Assassin's Creed III: Liberation.
Those who cross connect between the two games will have access too the following content on the PS Vita...
— Aveline can use Connor’s trademark Tomahawk — Special multiplayer character to give you an edge on PS Vita — Instant upgrade to Aveline’s ammunition pouches — An entire mission playable as Connor!
Connecting the two devices is fairly straight forward, but there's instructions on how to do so at the PlayStation Blog if you need help!