Hi stuckinthereeds,
The 20 gauge is a very good general purpose upland gauge. Built on a frame originally intended for the lighter gauge they can certainly be more pleasant to carry around all day! Ammo availability is as wide spread as for the 12, just as inexpensive, and can do just about anything the heavier gun can, (other than hunting really heavy big game).
I remember reading a line along time ago is some shooting publication. "A man that uses a 12 gauge is hungry and has a family to feed. A man that uses a 20 gauge takes pride in his "shooting". A man that uses a 16 gauge can't decide if he is hungry or proud. Oh, using a .410 is just plain bragging."
Good luck with your decision.
Dave