The 3.5" 12 or 10ga is really only an advantage with big steel shot for geese IMO. I own a BPS 10ga, and have owned 3.5" 12's. To me, the 10 patterns better and kicks less than most of the 3.5" 12's.
^^That^^
My ten works FANTASTIC. I puts steel T shot into a 30" pattern circle at over 90 percent. I now have a 3.5" 12, and it's not a back bored gun, it's a Mossberg 535 I picked up at a pawn shop lately. I have yet to purchase a box of steel 3.5" Ts to pattern, need to get off my duff and find some.
Regards to ammo, this is snow goose country. I live about 20 miles west of Eagle Lake, Texas and work in El Campo. The Walmart in El Campo doesn't have a lot of ammo variety, but they have 3 shelves FULL of 10 gauge steel T.
Oddly, they don't have any 12 3.5". I have a MEC press set up for 10 gauge and can load it for about half the Walmart price. I stopped bothering with 12 gauge reloading a long time ago, but a couple of years ago, 12 gauge 3.5" steel Ts were running 21 bucks and change while 10 was 25 dollars, that's a 25 round box. 10 is up to 30 dollars now at the El Campo Walmart.
BUT, I'm a goose hunter. However, my 10 is a H&R turkey gun. If I had turkey here (can't figure out why I don't), it'd be my gun with sixes. I have some sixes loaded up for it and they pattern well, too, out of the turkey choke my gun came with. I had to buy a modified choke for it for the steel. I am told this gun will work with Browning chokes, but I haven't tried it. Both my chokes are H&R sourced. This gun cost me a bit over 200 new. I just couldn't bring myself to fork over 700 for a Browning no more often than I book a goose hunt, but I'm kinda wanting one now that I know how well the 10 patterns. I picked up the 535 for 170 dollars, just couldn't find a reason to leave it in that pawn shop at that price. I'm hoping for 80 plus patterns, might use it in lieu of the 10, don't know. If it patterns down in the 70s, I can more justify looking for a BPS 10. I really like those Brownings, high quality firearm with great lefty ergos.
Oh, and about recoil, my ten is 9 lbs. I've fired 21 rounds in a morning no problem. The 12 is a couple lbs lighter. It might hurt.
It ain't bad with 3". I shimmed it for perfect fit and have a Limb Saver on it.