If you shoot it better than its a no-brainer. 20 ga is fine on decoying ducks. Jumpshooting the river may require the 12, unless you can get close enough. Heck I use a 28 in the blind sometimes with Bismuth. Most times from the boat though I have the 870 12 in my possesion. It's only a 2 3/4 3 " magnum but with the hevi shot 6's a 3" shell is plenty for me. Steel 3's do a number on dropping em but they don't fall dead.

Shoot what fits you and shoot it in the situation that fits.