Both the 16 and 28 are considered to be the "square loads" preferred by upland hunters. I know several folks who only hunt birds - clays might be a few rounds as a warm up - and they swear by the 28 for small birds and the 16 for the larger ones.....both gents have been hunting longer than I've been alive - way over 50 years - so who am I to argue with that?
Personally, IF I could find a 12 that "carries like a 20, hits like a 12" like the 16 - i.e. a 6 to 6-1/2 pound 12, that would merit consideration....
But if you can find a 6# 16 instead of using a 6# 20, then I agree - go for it