A lot of the decision is going to depend on you, your shooting and the environment. For a general idea of what to choose, stick with the mid price range standard round steel shot at modest velocity, say 1400fps give or take 50 to 100fps. Then, figure out what the furthest shot you'll be taking is and start patterning. Nothing fancy, just a well distributed pattern with few 4 inch holes (5 inch holes for your goose loads.) See if you can get a copy of Roster's lethality chart to get an idea of the minimum number of pellet strikes in a 30 inch circle for each type of bird.
I'll tell you what we use up here, but again, it doesn't necessarily mean it'll work best for you. An ounce and a quarter of #2 steel (Winchester Drylok, Remington NitroSteel, Federal SpeedShok or Kent Fasteel) at about 1400fps or so with a .020 choke works well on late season large ducks in our salt marshes, particularly when the wind is blowing. If the ducks are really cooperating and boring right in, we might switch to an IC or Sk choke.
For geese (large Canadas) again, an ounce and a quarter of BB steel is my own preference. As far as choke, sometimes the larger pellets don't like as much choke, so you need to pay close attention to your pattern work.
Most of our shots on ducks (blacks, mallards, gadwall and widgeon mostly) are from 25 to 35 yards. I make it a habit of not pulling the trigger past 40 yards unless the bird has been hit before that. Then, you do whatever you can to bring it to bag. The loads I listed above will pattern reliably out to 50 yards for that purpose.
I feel slightly sorry for a man who has never patterned his gun, who has no idea how far his chosen load will retain killing penetration. But I'm extremely sorry for the ducks he shoots at beyond the killing range of his gun and load - Bob Brister