You can buy light 7/8oz loads of 6s for a 12 that take some of the ouch out for farm and truck duty. And a pullover "Limbsaver" recoil pad is a great investment.
I love 20s but a 12 is a MUCH more versatile shotgun--all kinds of loads for all kinds of situations ("kills from ducks to Dukes" as a Serbian guide I once had said.). And SOMETHING that goes bang is always available--even in an "ammo drought".
My experience of shooting deer and piggies with a light 12 (Ithaca M-37 "Deerslayer") is that the pain only comes when you are sighting in. Adrenaline takes the pain out of an actual kill shot. Doubt that turkeys are very different.
