Just go with the 3” Affinity. I really don’t think the extra cost both in terms of money or recoil of the 3 1/2” is worth it. Wait till the birds are well within range and then hit them in the head.
You have to be a really good shot to make use of the extra few yards the 3 1/2” might give on paper.
Geese, graylags in particular, are big tough birds, and shotgun pellets won’t penetrate through to the vitals, especially at longer range. Trick to bringing them down dead is hit them in the head and neck, and indeed treat the head as if its a small or clay. A standard game load of No6 works perfectly well if you are patient and pick your shots.
Saying that I use an affinity with 3” high speed steel No 4 as I think denser pattern of the 4’s works better than bigger shot.
The Affinity is a brilliant and handles and soaks up recoil very well. The stock is also adjustable by shims so that you get it to hit where you look. If buying second hand make sure it has its shim kit.
The Affinity is pretty much a Benelli M2 with its simple inertia action which I have found very resilient to marsh and foreshore type Wildfowling. But do give a good rinse to get rid of the salt, especially in the trigger where there are a few coil springs that a bit of rust would quickly eat away.
The other gun I would go for, if you can find is the Browning BPS. They are very good guns, but like chickens teeth. The Benelli Nova is also good - had one for a while and really liked it, but mine was an early non adjustable stock and it shot too low for me. The ducks didn’t mind though.
Depending on where you shoot, I think a lot of wildfowlers err on the side of a big heavy gun and end up banging away at birds far too high out of range. Instead I think you need and easy to handle gun that shoot instinctively allowing to keep low till geese and ducks are right on top of you, then its an instinctive reaction at two or three before they get out of range. I always the best results are over splashes at high tide, or right down on the river bank at low and getting yourself into the flightlines. Then pick your shots. Don’t bother shooting at out of range birds and big skeins let go through and then pick of the individuals that are trying to catch up - they are usually lower and respond to a call. I think the affinity is superb for this type of shooting.
Don’t look at the Hatsans - they have a plank like handling by comparison.