The pre-order and other unlock bonuses go something like this, AFAIK:
Limited Edition: Immediate unlock of M1911 pistol, M1A1 Thompson SMG, tracer dart pistol, and several vehicle specializations (alt-fire, motion detector, improved armor)
Gamestop pre-order: immediate unlock of the AKS-74u and 30 days exclusive access to Squad Rush (now expired and SR is available to everyone)
Veteran (already played at least one Battlefield game): exclusive unlock of the M1 Garand rifle, and immediate unlock of the right-side gadgets for all classes (Assault: ammo box, Engineer: repair tool, Medic: medkit, Recon: motion detectors)
All new copies of the game: single-use VIP code, which lets you download for free the four DLC maps already released. If you buy used, you can purchase VIP status for $15 in the PSN store (or 1200 XBL points, I guess) There will probably be more free-for-VIP content in the future, but who knows?
So if the difference between a used copy and a new is less than $15, go for the new copy.