Depends on the freewheel and hub...but typically a hub will outlast a FFW. In my experience FFWs are more reliable though, and a lot cheaper to replace.
For a stock like the Zoo pitbull 18:15 or 18:16 should be perfect, the hub spacing should be 135mm
Freehubs like the Chris kIng are very good, as is the hope pro2 trials if you dont mind low engagements. I wouldn't trust any form of Deng freehub though