I hunted most of last season with a 28ga SXS. I shot about 125 pheasants with it using 3/4 oz and 1 oz of No 5s. I really couldn't tell the difference in the killing power in the two weights. But then I use one ounce of 5s in my 20 and 12GA guns as well. The smaller gauges just weigh less. I bought a Benelli Ethos 28ga with 3" chamber this summer. I'll use that gun this year too, but probably won't buy any 3" shells unless they are cheaper than RSTs with 1 ounce of 5s.
I would be interested in knowing whether or not you were shooting flushing birds in front of dogs or passing shots at distance with the 28. What chokes? I will be handling dogs this year in SD and I sure would not mind carrying either or my very light and comfortable 28s.
