Haven't read any of the other posts, just your question, but I can just about bet everyone says the same thing, and all I can say is get the 20 gauge and you won't ever be sorry you did.
With slugs mine shoots the size of a fist at 40 yards with Remington sluggers and the bead sight.
It's light weight but doesn't punish you like a 12 will in the same weight gun. It's not cumbersome at all in the woods and carries really well and easily balances when making a quick shot.
I like it as a my knock around gun that I can grab and go, and not worry about bumping a scope, and I can handle anything from deer, to hogs, to turkeys, rabbits, doves, and quail with an all in one gun, and a handful/pocket full of various slugs and shot.