Last year, I had a very good year on pheasants with a 20 gauge Ithaca Ultrafeatherlight shooting 1 ounce #6 copper-plated. I only noticed a drawback when I got past 35 yards. So late season, it seemed prudent to switch back to a 12 gauge. The 12 is definitely a better all-around choice. If you have a 3" mag 20, then the Federal Grand Slam load will make it mimic a 12 but with a lot of recoil.
Wild pheasants can be amazingly tough to kill with any sort of peripheral hit, and going-away shots require a lot of penetration. #5 nickel or copper works well, while #4 works even better but then the patterns start to lighten up if not using a tight choke. #4 copper through a really tight choke can make incredible kills at ranges to 75 yards, but chews up birds at normal ranges.
The biggest problem is that any sort of not-immediately-dead pheasant is going to run/crawl into cover and then underneath the grass and cattails, where you can never see them even if you are literally standing on top of them. That's where a good dog that will hunt "dead" is invaluable.
