A 20 gauge putting 10 pellets in a birds head and neck will have the same out come as a 12. The speeds are generally the same just less shot. , which in results is the same energy down range per pellet. 20s have tend to pattern tighter then a 12 in my experience if you spend the time get a good choke and loads. Ive seen muliply toms drop in there track at the 40 yard range with 20s.
you dont see many 10s getting used for turkey, most go to a 12, the people i know that go from a 12 to a 20 never go back.
Know guys that are starting to use them for ducks and really like them also