Despite all the cackle of the magic of the 28 gauge (hits harder than it should, etc.), there is a significant difference between it and the 20, IMO.
With the same shot charge - 3/4 oz - the 20 throws a better pattern, in my experience, than the 28. Go to 7/8 oz or 1 oz, and there's no contest. The 28 is pretty inefficient with the larger charges; the 20 is right at home. My favorite chukar load in the 20 is 1 oz of hard #7s, a beautifully patterning load. The 28 can't get close.
The big advantage transcends ballistics - the nice little guns scaled to the 28 are a delight. That said, I find my shooting falls off signficantly when gun weight goes below 6#, so my lightest guns weigh in right there - a weight that's very well suited to a 20 gauge.
The 28s cast a spell on us, no doubt. But let's not confuse that with logic...
