From a practical point of view a 12 is good can get cartridges anywhere near enough, can sometimes be hard to get a good selection of 20 or 28s
But if you want a nice gun with finesse then I don't think you can beat a 20 or 28 on a scaled action, they are just lovely to handle and are somthing a bit different, they are alot more elegant that a 12 bore over and under to use. Light, fast and very pointable.
Not saying there isn't some nice 12s about but if I had my pick purley for driven pheasant and partridge it would be a 20 bore. If it's a mixed bag with the potential to throw some steel at ducks and geese it would be a 12.