No question the 20 is more versatile than the 28, and the 12 more versatile than the 20--and that you can find 12's that don't weigh much over 6# if you look hard. (A friend with a Red Label 28 thought his gun was light; we put it on a scale, then my Army & Navy 12 with 28" barrels. One of us won by an ounce, but I can't remember which.)
And you can indeed find 20's that are lighter than many 28's. I think Guerini has an OU out that's under 5 1/2; Franchi 20ga OU's tend to be in that range as well. Which puts them lighter than my Parker Repro 28 by a few ounces, although my Repro 28 is half a pound lighter than my quite light Army & Navy 12, and when in the grouse woods with lots of one hand carry situations, I'd just as soon save that half pound.
The 28 is definitely a gauge where reloading pays off, big time, because as pointed out above, primers cost the same for all gauges; you use less powder and shot (which is the same price, no matter what size hull you put it in), and there's almost no difference in wad price at the club where I buy them. In other words, I can reload 28's for pretty much the same cost I can reload 12's. (Hulls are somewhat more expensive, buying them either once fired or shooting up factory shells, but that's pretty much offset by the savings in powder and shot.) And I'm not shooting enough factory shells to make anyone rich, that's for sure! So why not a 28? I mean, so what if the gauge is a little odd? So are guns that open via a sliding breech rather than a toplever!
