i personally would use 2's or bigger for geese, 4 for late seaon ducks, and i use 6's for ducks over dekes of jumpin (close range)

but..........if you REALLY only could buy ONE kind, i'd get 4's and keep my shots at the geese good and close

my .02

ps, as far as range goes, 30-35yds is perfect, 40 is pushing it, after that i think the risk of wounding is too great. work em close!