In my experience a 12 with lighter loads/reduced recoil loads kicks less than a 20 with magnum loads. There's a much wider variety of loads available for the 12 than for the 20.
Recoil is a matter of physics- the more you push out the muzzle and the faster it goes, the more the gun will push back. The more the gun weighs, the less it will push back, and most 12s are heavier than most 20s.
Handling recoil as a shooter is for the most part a matter of proper gun fit on the one hand, and proper form and a good gun mount on the part of the shooter on the other hand. A premium recoil pad can help, but good form and proper fit are primary considerations.
Making the decision as to which gauge to get is completely up to you, but if you are in good health there shouldn't be any reason you can't shoot a 12 comfortably if you want to. I only asked the question to dtr and determine why you were looking at a 20, not to cause you difficulties in making the decision. Try shooting a 12 ga. 870 with light field loads and see how it goes. If it's manageable, then try it with standard velocity buckshot or slugs. IMHO it's better for a person to make up their own mind based on their own experience, and it's better to try before you buy.
FWIW,
lpl