Gauge has nothing to do with the size of a shot pattern. It may have an effect on the density of the pattern, but even then that's determined by the amount of shot in the load. Ie: you couldn't tell if a shot pattern was fired by a 20 or a 12ga. if both were choked identically and fired the same amount of shot.
I personally would go with the 28, but I've used mine for thousands of shots at skeet and sporting clays so I'm prejudiced. One thing to consider is the general availability of 20 gauge ammo vs. the spotty availability of 28's. Something to consider when you've driven 200 miles to hunt and discovered your 28 gauge shells are back home on the porch and all the local hardware store has to offer are 12's and 20's. Don't ask me how I know.
