Barrel length is your own prefference the newer guns get great performence out of short barrels. My brother is in to duck huning big time. He loves the shorter barrels.
If your buying a gun make sure you get one with a removeable choke most all newer ones have. Duck hunting you will probably want amodified choke shooting steel shot. Lead shot is illegal over water. Turkey hunting you will want full or extra full ckoke pending how it patterns.