If I am not hunting with a good dog I use a 12 ga Beretta OU with 7 1/2 shot for most days. Early in the season I put in a skeet choke on the bottom and IC on the top. Later in the season when the leaves are gone it will be Mod in the top and IC in the bottom. Late in the season I put a load of #6 in the top if the wind is up at all.
A 20 ga is a little slimmer and a little lighter but you are well gunned with your 12. The balance of the gun is more important than the gauge to me.