To me at least part of the appeal is the (foolishly incorrect) perception some of acquaintances have that the 12 gauge is a real Man's Gun, hard-kicking and brutish.
I show up with a 12 and they all think I'm trying to prove something.
I'm not a big fan of the 20. Had one for a while. My first gun, in fact. Seemed a little anemic. Although my Benelli is a great field gun, I'd be more inclined to keep that one for the range and get a 16 gauge A5 for a field gun.
So, yeah, my inclination towards the 16 is partly an emotional reaction, not only to go against the grain, but also to not be perceived as trying to prove something by toting a 12 around. Silly of me. When I can afford the shells, I put 300+ rounds/week through my 12. Getting a 16 to have a slightly milder load *is* silly. Partly it is general nostalgia. I want an A5. 16 just seems like the right gauge for such a historically important shotgun. And partly it is more specific nostalgia: my grandpa shot a 16.
As for ammo, well, I'm picking up a MEC single-stage press to learn on and get started with 12. When I find the funds for a MEC 9000, I'll also get the kit to convert the single-stage over to 16.
And this may be silly, but I jsut like the feel of 16 gauge shells. 12 seem kinda' fat and oversized. 20s seem kinda' small and wimpy. 16 gauge high-brass shells are just the most aesthetically pleasing shotshell it hase ever been my pleasure to handle. Weird? Maybe. Ask me if I care.
I think it'll stay around enough that I'll always be able to find some hulls to load. Of course, I'm talking a big show for someone who has yet to actualy reload a single shotshell, but ya know how that goes. I've been thinking like a reloader for months now, with the saving and the sorting and the prepping, it's just a matter of getting the gear.