3.5 inch 10s just flat outkill 3.5 inch 12s. Always have. The 3.5 12 is like a 3 inch 410 IMHO. Works ok, but there is better out there.
The cost difference of shells isn't enough for me to worry about vs how many get shot a year, a couple of cases of shells of 10 ga wouldn't start to even cover my fuel bills for the vehicle, never mind the airboat.....and lodging and food..
I only have a BPS 10 at this time, but I can consistently kill furhter with it than with any 12 I've shot.
I'm in the same boat.. which semi 10 to buy? All the research I've read says SP10 by a hair over the Gold 10. Bottom line both seem like they'll have hiccups but do fine. I have a well beaten 11-87 that everyone says is junk, but its never missed a lick yet and I had a misfeed twice the other weekend with an 870, but never the 11-87. And I've seen a few benellis go down in teh field and or need to be cleaned while hunting with them so I'm not sold that they are the end all either...
Is a 10 big and heavy, yeah, but not so much that it really makes a difference to me.. my BPS has a short barrel and that makes a lot of difference over the 30 inch it came with....
The question to the end of this... do you REALLY need the 10, and if so its by far a better choice thna the long 12s. But in reality most all the hunting most folks do can be covered by a 3 inch 12 of your choice.
As far as ammo availability, I'm not one to be looking for a few boxes, I either load or buy by the case for each year.