I agree that the 20 gauge is just about as versattille as the 12, but if i was a long range wing shooter i would still pick the 12, or maybe even 10 gauge.
Yes, when I was waterfowl hunting, the 12 seemed just about right. That was when lead shot was still legal around here. Steel shot is a big no-no in a thin barelled double. Were I to do it again, I'd think about a rugged 12 (maybe a Mossy or Remmy) with a 3-1/2" chamber. A 10-gauge is just too heavy for me.