Having used both the 12 & 20 870 (and loving both) in my narrow minded opinion you can't really go wrong with either.
I almost perfered the 20 just for lack of shoulder bruises.
However, here is something to consider: What is the gauge the the others in your party hunt with?? If you are on a long drive, hike, walk, stalk and get caught a few shells short (doesn't matter how you plan it seems, everyone at one time or another gets in that position I would assume) Can you count a a partner or family memeber to cough a couple of shells to get you back?
I was shooting Magumum 3's, rifled barrel on my 12 last year. All it did was cost MORE cash and hurt my shoulder. After the season I re-sited with 2 3/4's, the same as everyone else in my family uses. There may be SOME difference tech wize, but I am very pleased to be back.
So, I suppose I didn't really provide you with the answer that you were looking for but you really can't go wrong with either.
(the barrel I bought for my 12 was about 40 bucks cheaper than the 20)
