I hunt with a 20 ga Remington 870 Express Magnum chamber using 3" slugs and I've never had a problem taking a deer down with 1 shot. I got the gun for Xmas when I was 12 and don't have enough money right now to replace it, and haven't had reason to!
It is the combo with the interchangable slug and shot barrels, and I'd rather carry a 20ga pheasant hunting than a 12 anyway! With 3" shells it packs about as much power as a 12 with a little less weight which can make a huge difference on your arms when you're walking through brush 10 hours a day!
As far as aim and shooting straight, I dropped a doe at 80-90 yards this past year with 1 shot while she was walking. I'm a firm believer in the "It's the indian not the arrow theory" though.
My dad always hunted with an old sawed off 12 ga until it jammed on him the year after I got my gun. Since I was at school, he took my gun back out for the afternoon and dropped a doe 1 shot at 100 yards. He's a better shot than I am (99% or better marksman score in the Army on every rifle he qualified on), but he was sold on the 870's and went out and bought a 12 ga 870 for himself that Xmas. He's had great success with it as well.
I guess what I'm saying in an extremely tangential and circumstantial manner is get the gun that works for you and practice with it. Remember it's the Indian, not the arrow.
If you don't mind the extra weight and want a bit more power, go 12. If you'd rather sacrifice a little power for less wt and a little lower price, go 20. As far as I'm concerned, I don't care what gun I have with me as long as I'm used to it any day hunting is better than the best day at work!
