I had something of an epiphany the other day and it's taken me almost 50-years to get to this point (I'm slow, I know, & I understand that this will rub some folks the wrong way) but IMHO...most sub-gauge guns are superfluous. 16s are the very best of them, but they're all a significant compromise past that point. In doubles, a heavy 12 (~8lbs) will do anything that a 10 will do, and perhaps do it better (more load options, easier to carry). A light 12 (~6lbs) will do anything a 16 or a 20 can do and will be even more efficient when it comes to killing patterns. 28s and .410s are fun and even "cute" but largely unnecessary (unless you're into self-flagellation). If you've smaller or have developed some physical limitations and a heavier gun isn't much fun anymore, then by all means shoot a .410 but...you better be awful good with it, otherwise you're just wounding animals unnecessarily.
There is a reason that more 12s are made than any other gauge and it's because they are "the Alpha and the Omega" when it comes to utility and lethality. History has proven that fact over & over again. You wouldn't want to use the 8lb gun in the uplands (unless you had too) and you won't enjoy shooting the 6lb gun on targets because each has its place where it excels.
I have been giving this thread some thought (thank you for that, I rather enjoy pondering things that I had not considered before) and figured I would share some of those thoughts. These are NOT a post of "facts" just ramblings that, like the OP, is to get feedback, yea or nay. If one is on a tight budget, certainly a single (or in the case of the OP 2) guns that can "do it all" is desirable and in that vein, there is no question that the 12GA reins king, if for no other reason than cost and availability of ammo. I'm not really buying the "wounded birds with subgauges" argument because that is 100% a function of the shooter, not the gun in many cases. Most .410's. for example, seem to come in full choke. If the shooter is proficient enough to hit a bird with the centre of the pattern, a full choke .410 will bring down an uplander just as efficiently as a 12GA with a more open choke since with an open choked 12 shooting 1-1/8OZ for example could have similar pellet count in the same circle as the full choke .410. Perhaps I will pattern a FC .410 and compare to a modified and IC 12 and do a pellet count to see if this hypothesis holds water or not. HOWEVER, the 12 GA with the more open choke leaves a LOT more margin for error with the wider pattern. On the flip side, shoot a grouse at 20 yards with a 12GA full choke and, while it is unlikely that it will be wounded, it is equally unlikely that it will be edible (if hit with the centre of the pattern). IMHO, sub gauges are Wonderfull BUT require more skill to use effectively. For the record, I am NOT a highly experienced shotgunner like many here so these are all just "thoughts" with not a lot of field experience to back them up. I posted this to get the thoughts of those more experienced as to whether or not this makes any sense. Interestingly, I was having coffee with a buddy that used to be a trap shooter. He has a 22GA French flintlock SXS. Said before buying it he took it to the range and tried it out in a round of trap and hit 20 (I am assuming that is out of 25, I have never shot trap before) and that is with a 22GA cylinder bored. In the right hands, I believe that the sub gauges can be just as effective on most stuff as a 12GA but more pleasant to carry and shoot. Even a 12GA will wound if the bird is only hit by the fringes of the pattern. In closing, it seems to me that sub gauges can be just as effective as a 12GA but require more choke, closer shots and a skilled handler. There WILL be exceptions of course (hunting geese come to mind) but would like to hear if any of that makes sense or am I overlooking or misunderstanding something. Flame away.
