I would say go with the 20g, but the most important thing is to shoot the gun you shoot best. Any gun that puts the pellets where you want them will yeild the results you desire, which is birds dropping from the sky. 20's are nice. More often than not I carry my 20. I have also taken a lot of game with my 28g, pheasants included.

The key is having a gun that fits you well. One easy way to judge if a gun will fit you is to mount it with your eyes closed, then open your eyes. If you are seeing the rib you are ill fitted. You want to see just the bead mounted on the muzzle. Seeing the rib you will shoot high. Thisusually means the gun needs more drop at the heel to fit you. This is of course a really basic explination, and I'm not a gunfitter, but I'm sure there are members here who can explain it in more detail.

Contrary to popular belief, a 12g doesn't hold more shot than a 20g. Shells are loaded by weight, so an ounce of shot willbe the same amount of pellets regardless of which hull they are packed into provided you load both hulls with the same size shot. The difference is that a 12g hull can be packed with heavier loads should one choose. However, a larger load doesn't guarantee better performance, or better balistics. To determine which loads work best on moving targets you need to shoot a lot with different loads. IMO if the gun fits well, and you spend a fair amount of time on the skeet field 1oz, or even 3/4oz loads will perform fine.