If I hunt Tundra Swan or Canadas, I'll use the 3.5's but I've killed more ducks with a 20 ga than a 12. I used to use a Browning 3" A-5 20 ga with lead when it was legal. Went to 12 gauge 2 3/4 steel and then went back to 20 gauge steel #3's in my Ruger O/U. Right now I'm shooting my SBEII with 3" just because I have a bunch of 12 ga.
To tell the truth, based on the number of them that I see on the shelves right before Duck season vs the lack of any on the shelves halfway through the duck season it would appear that the most popular duck load around here is the 12 gauge 3" double X turkey loads by Winchester in #6 or #4 shot. Always wondered why they stocked so much turkey loads in the fall down in the marsh when Turkey season isn't until the spring-and I ain't never seen no turkey in the marsh no
For decoying ducks all you need is a 20.