ORIGINAL: Flairball
Bring the gun you shoot better. The idea is to shoot birds, right? I prefer a 20, but have a 12 I us for waterfowl. On upland I haven't seen any difference between a 12, and a 20, and for a long time I used a 28 for all my upland hunting. An ounce of shot holds the same number of pellets whatever the guage. Just put the pellets where they need to be and the birds will fall. Have fun.
Agreed on bringing the gun you shoot better, but there's a reason the 12 is manufactured -- thick cover, longer flushes and as you have mentioned, hunting waterfowl.