I’ve used 20’s and 28’s as well as 12’s my whole life. And now have picked up a 16 again also. All will take pheasants. The quality of shot shells has never been better.
Perhaps much more is dependent on you, your inclinations, desires; a favorite gun, pointing vs. flushing dogs, etc.
Frankly, in fifty-five years of hunting pheasants, I’ve not discerned a difference when carry a 12 or a 20 other than size and weight.
