There's two kinds of recoil:
1. Real recoil (objective).
2. Perceived recoil (subjective).
Yes, the first is pure science and is the same for any given gun/ammo used, but the second is all over the map due to each person's perception of a given gun's recoil. Some small people handle recoil fine, some big guys are bothered by it. You never know.
In the real world -- and science aside -- shooting a 12-guage and then a 20-guage, the 20 is easier on you. A 410-bore would be even more so than a 12.
I'm not sure for what purpose (mission) the OP wants a shotgun but if he is concerened about recol, there are ways to reduce it he can apply to a 12 if that's what he wants.
I would go ahead and get the 12 (or shoot someone else's), try it to see how he perceives the recoil, and if it's a bit harsh for him, try assorted recoil-taming techniques such as different ammo, a recoil-reducing stock, add weight to the stock, shooting position, etc..
EDIT: For current example, see this guy's post:
http://www.thefiringline.com/forums/showthread.php?t=255073
There's certainly nothing wrong with a 20-guage, but if the end goal is a 12, he may as well get a 12 right up front.
-- John D.