I do alot of waterfowl hunting, at Long Point, and at various other local hot spots. If you have a good decoy set-up and are good at calling them in, then a 2 3/4 is really all you need. With that being said, I use a 870 supermag, and absoloutely love it. Ya, I can call in lots of birds, but I don't have to only settle for that when I am shooting 3 1/2 Black Clouds #2 with a full choke I have folded Mallards from 60-70 yards. It seems almost unbelievable sometimes.
Either way if your a good shot and have a nice decoy set-up, then you will have no problem filling your bag limit. I just find that if the ducks are getting smart later in the season, and they wont get close to your set-up then you can usually reach out and get them with a 3 1/2.
One more advantage is if you have duck loads in, and along come a flock of geese, your all set. I have dropped more Canada Geese, sitting in a Blind duck hunting with #2 or #4 shot then I have when I was strictly hunting Geese with BB loads. The last outing of the year for me, I was hunting Mallards, and along came a Flock of 8 Canada Geese, and I shot a Tripple of Geese using #4 shot.
I spent about 3 hours that day cleaning 6 Mallards, and 3 Canada Geese. Had a good dinner Christmas eve.