#3's are my all around favourite, but since of the birds I shoot in BC are puddle ducks decoying, #4's work fine and if I could get some 6's I would, but I pick my shots!
As for 20's there much fewer choices then 12. I shoot 7/8oz of #4's, which means I'm putting out about 2/3rds of what I would with my 1 1/4 oz 12ga shells. I shoot an older gun, so going to 2's is a bit of a concern in that they don't move as much in leaving the barrel as 4's and there is a slim chance of choke issues, but I'd rather be safe. There are a few makers of 1oz loads, but they are pricier then the ones I've been getting by a big margin.
The real trick with 20's, well any shotgun is set up to get the birds in close, and then pick your shot and make it count!
Oh, and the #1 thing to do to killl more ducks...
PRACTICE, PRACTICE, PRACTICE!
Very best,
Ian