This all depends on what type of hunting. If you are walking a little or basically positioned in a field, then the 12 works fine and is the choice for trap, skeet and sporting clays.
If, however, you are doing a lot of walking or climbing of hills, the lighter 20 will prove to be the better choice, as they typically weigh at least a pound less.
My target 12 is 8.5#, my upland 20 is 6.5. Which one do you think I prefer for sporting clays and which one do you think I prefer for upland birds?