I own several of both, and really kill no more partridge with the 20 than I do the 28.
The thing about the 28 is the better ones are built on a lighter frame, so carry better at the end of the day.
The downside is when shooting 7/8oz loads, the 20 works better and the guns are a bit heavier , so they recoil a bit less.
however, a 3/4oz load of 8's going about 1300FPS will bust clays and kill any bird within 20 yards!
Cat

scopes are cool, but slings 'n' irons RULE!