There are those that use a 20 gauge for waterfowl and yes they will kill ducks/geese fine within a certain range. However, in my personal opinion, I too would go with a 12 for waterfowl and 20 for upland. However, as already mentioned, there are some nice light, great handling 12 gauge sxs's out there that are wonderful for upland. Plus you don't need to use heavier, 2 3/4" loads for upland in the 12 if you don't want or need to. You can buy lighter 2 1/2" loads. Of course these might not work in a semi-auto but I'm not sure what type of gun you are after. Bottom line, if you have only one gun I would go with the 12.