The 28 has a certain amount of coolness to it but IMHO there isn't enough difference in size to justify the $$ of the shells. Also the size of the guns are often not much. For example I have a Browning model 12 20 ga. I was comparing it to a 28 ga model and found the 28 was actually several oz. heavier. As to killing power there probably isn't that much difference in the loads to notice much.
