I have had extremely poor success patterning anything larger that #4 steel out of a 20 ga... That being said, a 1 oz load or a 7/8 oz load going over 1400 FPS is a 40 yard killer all day long if you can put the pattern on them (front end of the bird)... If you are shooting at birds over 40 yards, take the 12 ga... In fact if you think you are going to consistently shoot birds over 35 yards, the 20 ga. is probably not the best option shooting steel shot.
As for which steel load shoot the best, that seems to vary widely between different gun and choke combinations.
If you have the money, I know that Hevi-Shot in #6's will out preform just about any steel load in a 12 ga. for ducks. And surprisingly, #4's will work well with geese, and for those windy days they make a great duck load as well.
As it turns out, I have switched to steel #4's and #3's in the 12 ga... I tend to shoot in front of birds when I miss, and I am finding I get a lot more head shots with those smaller shot sizes. If you stick with steel, I would go with #4's...
