where geese are known to roam...
I was going to say that the 3 1/2 should only be considered when you specifically go after geese a lot.
I've hunted ducks & geese most all of my life in the lower MS flyway - in the delta and in farm country, on big water, and in the woods. Most of the hunting I have done has been for puddle ducks (Mallards, Pintails, Gadwall, Teal etc.) over decoys with calls, and amongst the people I've hunted with the most, pass shooting ducks is about as sporting as running down armadillos with a sportscar. There are some species of ducks and some locations where pass shooting is all there will ever be, but most of my cronies would usually pass on those in favor of hunting over decoys for proper decoying ducks that respond to the call. As such, almost all of our shooting is at closer ranges, and the only long range shots ever taken are to try and bring down an obviously wounded duck.
We use mostly 2 3/4 shells, and it's common to load a pump gun with 2 x 2 3/4 and a 3" for the third shot, because shooting a flaring mallard in the hind end requires a little more oomph. When there are two load weights in shell length - e.g. 1 1/4 oz vs. 1 3/8 oz loads in 3", we always shoot the lighter weight load on the assumption that it might be faster and knock down better. No scientific proof. Typically we shoot #2s or even BB's at ducks, depending on what kind of place we are going to.
As to geese, in some of the places I have hunted it's not uncommon to see far more geese in a day than ducks. Very common to see geese in the tens of thousands in a day of hunting. Most geese are out of shotgun range no matter what gun you have, and stay that way, although depending on the weather or good luck, you may get some decent shots at them some days. We've killed as many as 25 to 35 in one morning, although toting that many geese any distance isn't fun - and neither is plucking them. Most of the people who opt for the 3.5" 12 or the 10 ga. down here hunt in the Delta where they commonly see thousands of geese every day, and have shot at many geese out of range with no success. I guess the 3.5 extends your range somewhat, but it has never seemed worth it to me. Geese can be pretty hard to bring down with steel shot, even if they are in range, and in the old days, people sometimes shot at them with 3" mag #4 lead buckshot. I've seen my best friend pop one that I wouldn't have shot at with a 870 with a cut down 20' barrel and 3" #2 shot - so it's partly a matter of good shooting and partly hitting them in the head.
Anyway, in many years of duck hunting, I've never really felt much need for the 3.5, and they never even had it for most of my years... It has a longer receiver, and a longer stroke, and I guess is a bit heavier too, all of which are undesirable to me. So unless I was a dedicated full time goose hunter, I would pass on it.
As for type of shotgun, whatever you get will require a lot of care and cleaning, because proper duck hunting is hard on a gun. Plenty of water and mud. Mine has been underwater numerous times, and covered in mud, dirt, etc... cleaned with a waterhose on occasion. Most of my hunting has been done with an 870 Express that I paid $175 new for, and my best buddy has the same gun, although he paid $50 bucks for his because it had a blown up barrel which he cut to 20" and had the chokes reinstalled. They have served us well. Some of the guys I hunt with are Browning Auto-5 ists (that's a religion), and they spend more time cleaning and tinkering with theirs than you would with a cheap pump gun, naturally. One of my friends has a BPS, which I dearly love to shoot, but it malfunctions more often than the 870's. Another of my friends has a well used Beretta Auto - which is absolutely a Cadillac of a gun and has proven itself time and again, is a joy to shoot and easy to hit with, but also costs lots of $$. I know people that have the Benelli as well, but don't have personal experience with them. I have an older field grade Citori which I carry occasionally on fair weather days, and I like it, but they are even more $$ now.