20ga is not alot different than 12ga untill you get into the high loads.
Just depends on what you want
We used to shoot Pheasants with a 12ga 3 3/4 1 1/4 #6s a very effective load.
Now i only shoot preserve pheasants and i can kill them with a 410 2 1/2" shell #8 shot and a 1/2 load
Still all Pheasants now in the Dakotas are released birds no matter what anyone says