There's not enough information, Doug, to give a real great answer.
'It depends' ... what are you shooting? Teal & Woodies on a slough? Decoyed Mallards on a riverbend or cove? Pass shooting big ducks and the occasional goose?
Geese mostly?
Bullets matter more than the gun. I surely like Bismuth, but I handload, and I shoot wood and blue-steel, fixed-choke SxS guns.
If you're going to shoot Steel, any pump or auto that will handle 3" will do you for 95%+ of waterfowl shooting.
3½" 12ga is an answer to a question that should never have been asked.
You probably have the ONE gun you need in your closet already: an 870. ONE load for 85-90% of what you'll shoot: 3" 1oz of #2 steel that starts off at 1400fps or faster. The remaining 10-15% are passing geese, and 3" 1¼oz of Hevi- is sovereign medicine for them.
Bob