It comes down to the difference in the guns. If the 20 is light and fast you make up for a bit patchier pattern. If the 12 is heavier, it may be slower. Because of injury I used a light open choked LC Smith 20 late one year because it was the lightest gun I had. Killed wild birds in the snow as well as my 12's but it was easy to over power the swing and stop. Either over pointing dogs will work if you do your part.