In my opinion the PDW is the best in the pistol range, although a M9 SD with enough ammo is worth its weight in gold. The PDW can use pretty much all 9 mm clips (probably not the Bizons', but since that's not implemented in game yet it's not an issue), including silenced clips, this gives it a lot of versatility especially since ammo is always needed.
I'd just found a M24 to go with the M4 SD, but the tents have bugged out, so now I have to cross my fingers and hope the tents spawn on the server after its been reset...