I’ve seen 6’s work well on decoying mallards. Never used them myself. When cleaning those birds the meat was full of shot that barely penetrated. I use a few #4’s but #3 gets my #1 vote for ducks. I shot a few #2’s when mallards are the main target.
My understanding (not gospel) is that Delta focuses primarily on Canada. Canada is where many of the ducks breed. Many of the Canadian contacts I have trash talk DU. Like my drinking preferences (whiskey & beer) I support both. I’m stuck on which organization is beer and which is whiskey.