other steps
Hey Buck.
I reckon you realize that a day in a duck blind cranking off multiple rounds with a mag 12 is more punishing than a single well squeezed round at a gobbler.
That said, if you haven't had one installed, make sure your 12 has a top quality pad like the "Kick-eeze" or a "Limbsaver", they really do make a felt difference. Also, I'm a big believer in some type of sights beyond the basic bead on a gobbler gun.
At 70, you are entitled to hunt with near anything you choose, I hope I'm still chasing gobblers that late in life. Good for you. If you choose a 20, I'd invest in a box or two of the "space shot" loads, and I'd use a gas driven auto. The gas gun will kick less, and the entire rig will be a tad lighter than a 12. And I'd limit my shots to 30 yds or so, as eastbank has been advising.
A 20 is just not a 12, if loaded comparatively. Apples to apples and all that. Meaning, space shot v. space shot, lead v. lead, etc. Tight 1 oz (or less) patterns are all well and good, and good indicators of potential performance, but a pie plate on a tomato stake in the wide open off a support, is not a gobblers head on a bird with cat like reflexes, and a screen of brush or twigs that you didn't see in the heat of the moment, or because the light was marginal. And don't forget the 5 more yds or so the shot turns out to be cause you estimated wrong. Cause of all that, I will continue to hunt gobblers with a 3" 12.