now keep in mind that I hunt SOLELY with a 16ga.
1. The shells are a little more expensive and not all stores carry them. I have been able to find plenty. WalMart, Dick's, numerous small chains around where I live, etc all carry 16ga shells. Typically there will not be as much variety though.
You can also order online.
2. Depending on which gun model you have, the 16 will be lighter than a 12. My gun of choice is an Ithaca 37 and the frames are gauge specific. The older ones are wonderfully light and easy to carry.
3. The 16 does recoil a bit less than a 12. I do not think a 12 recoils bad though.
4. 1 oz of shot at x fps is still 1 oz of shot at x fps regardless of gauge, but the 16ga has had some dimensional advantages through the years, but modern loading techniques does negate some of that. the advantage was in the height to width ratio of the shot column.
5. if you use Federal ammo, you get to use cool purple shells...........
6. there are situations where a 12 has advantages in payload and shot selection. I would not want to shoot trap with a 16 unless I bought a reloading tool, and that would take a lot of shooting to recoup the expenditure.
there are also situations, such as waterfowl hunting where 12ga ammo has a price advantage simply due to the advantages of scale in production. we just have to face it, in today's shooting world many more 12s and 20s are made than 16ga shells.
But if you simply shoot a box or 2 a year during hunting season (if that many) the price difference is negligible.
I use mine for rabbit, birds and squirrels.
As far as HD, there is definitely going to be more and different ammo available for the 12ga. The availability of shorter barrels so you don't have to cut your hunting barrel (longer barrel being more difficult to maneuver in the house) is better, much better in a 12.
Maybe one of the folks that know about 870's can jump in and talk about the basic platform, whether it is a 16 on a 12 frame.