From the standpoint of strictly dove & quail & comparing the 3/4 oz vs the 7/8 oz loads the major difference will be noted at the Check-Out Counter if you shoot factory. If you reload there is some advantage to the 20 as well in a more readily available line of components & empty hulls. This is not to say there is a scaricity for the 28, just more for the 20.
Personally if one has a desire or need to shoot heavier loads than 3/4oz I would always pick the 20 over the 28 & yes I am aware the 28 has been loaded up to 1oz, but "WHY" has always been a mystery to me.

Miller/TN
I Didn't Say Everything I Said, Yogi Berra