Are 3 1/2" shells needed for waterfowl hunting? NO.
With that said i would say go ahead and get a 3 1/2" gun. you dont have to shoot 3 1/2" shells in it of course. My Beretta Xtrema II shoots 3 1/2" but my duck and goose loads are Kent Faststeel 3" 2's. 3's if there are alot of teal around. My Beretta is 12 gauge but it is built on a 10 Gauge frame so it can handle any load from low base upland game loads all the way up to the 3 1/2" mags. I dont know waht your budget is but I highly recomend that you look into an Xtrema.
I suggest getting a 3 1/2" gun because you might want to hunt turkeys or predators and you could use 3 1/2 inch shells for those aplications.
It would be hard to get better advice than this.
I have filled a many a freezer with my old Remmy 870 WIngmaster 3" that was my first gun I bought myself. That must have been around '85 or so? I got to the point that I didn't want to scar it up so I decided to get a new bird slayer.
I have a Browning Gold SG 12g 3-1/2" and a Beretta Xtrema II in 12g 3-1/2".
The Beretta is the one I grab when I am headed to hunt ducks or geese.
If there is a better handling, better shooting shotgun I have not found it.
I use 2-3/4 and 3" steel in #2, #3, and #4 for ducks as most of the shots are less than 20 yards where I hunt most of the time.
Woodies are the most common killed so a big shot isn't called for.
If geese are on the menu then 3-1/2" BB's and BBB's are what I use.
Everyone I know that has a Benelli, Franchi or Stoeger do not like shooting the bigger heavier shells in them because of the recoil.
With the Xtrema II there is almost no recoil.
The Stoeger is a nice gun for the money but for more money you can get a better gun.
The decision is yours.
If you are close enough I would be more than happy to let you shoot it and see what you think.
BigDozer66