This is my philosophy... I have made some incredible shots with a 3" mag. Like pushing 80 yards. When I got the goose, I noticed the only reason it went down was cuz of 1 BB finding its head. I have shot at geese from pretty close other times, seen feathers fly, and watched them fly away.
My thoughts on 3.5" are, even with the same amount of powder, the more BB's there are the greater chance of one finding a kill zone area.