I"m always asking: Whatcha gonna do with the shottygun? Defense; deer-hunting, bird-hunting? If primarily birds, ducks/geese, or dove/quail?
A very lightweight 20 will kick as much as a heavy 12, but most folks never notice recoil when shooting at game, and certainly not in self defense. You mostly notice recoil in, say, skeet shooting.
For in-the-house defense, a 20-gauge load of #9 skeet is as good as anything, and I'm assuming you'll develop the appropriate skills...
For dove and quail, personal skill is more important than gauge. A 20 will do just fine.
For ducks and geese, the ranges tend to be longer, larger shot are used, and by and large a 12 is better than a 20...