I think 20 gauges are just as accurate as a 12 gauge, and at close range both gauges produce about the same energy. The weight and the length of a 20 does have it advantages.
When it comes to recoil, 20 gauges are alot less punishing. But when your in a hunting situation you don't even notice the recoil, and you'll usually shoot once or twice. Today's 12 gauge slugs are getting smaller and lighter due to the want for a slug to shoot 200 yards. Compare a 1 3/8 ounce slug which is around a 600 grain bullet to a 260 grain slug wich is not uncommon for a 12 gauge slug today. A lighter slug is less payload which is less recoil.
I shoot a 20 gauge 870 with fully rifled barrel. I've shot 2 brands of sabots and with both I got 3 inch groupings at 50 yards. With the 7/8 ounce slug I'm shooting now, at one hundred yards the slug drops a drastic 8 inches below the bull even with me sighting the gun 2 inches high at 50yds. I don't plan on taking 100 yard shots maybe 80, but if your looking for flatter shots at longer ranges, I would go with a 12.
good hunting
jason