I try to duck hunt several times a week. I have pretty much stuck to shooting Kent Fast Steel, 3" BB's. It's a good all around shot, so you don't have to change loads if geese come in etc. If your an accurate shooting in the 40 yard rang and not a sky buster BB's make a large enough hole the criple loss is minimal. I used to shoot 3", 2's when the bird were consistenly under 20 yards. Nice full pattern with a modified choke. Minimal cripples.
A good thing to do is grab a few different loads and chokes and see which ones pattern the best with the least amount of holes in your pattern. Then stick with that load all season if possible. You will become a consistent shooter as you get used to shooting a specific load and shot size. Varying shell speeds won't be a handicap.