I asked this for a 30-06, now time to ask it for a 12 gauge. The model of 12 gauge is a Mossberg 500 Wood barrel combo w/ scope. Now when I say comparison I mean like, say the 20 has 1 recoil then the recoil of the would be this much more. The 20 gauge would be using some fiocchi target loads. So would the 12 gauge.
A light 12ga can be unpleasant to shoot for a lot of people. With 3 magnum deer slugs, it can be downright punishing for a new shooter. It's something you get used to though, and even start to enjoy it. This is totally subjective, but with a .22 being a 1 and a hot large game load out of a Ruger #1 being a 10 out of my own personal experience meter, I give the light 12ga an 8.
If your asking to go with a 20 gauge or a 12 gauge, i will say this. The 20 gauge is more practical for fun shooting because the 20 gauge will not beat your shoulder up as much as the 12 gauge, and will deliver the same effect on an intruder in slug form. If your looking for a game load the 20 gauge will offer a more enjoyable day of shooting if your shooting frequently. So to answer your question, the 20 gauge compared to the 12 it really depends on the particular load your using its it a slug or buck shot the 12 is going to be worse on your shoulder any way you look at it. But unless your just in it to say i have a 12 gauge then get the 12 other wise I strongly recommend the 20 and its also a good gun to pass down to your kids if you have any some day.
20 Ga. auto-loader is going to have much less recoil (in your example 1). I would say the 12G is going to have a felt or perceived recoil in the 3 or 4 range. My dad gave me an Ithaca Featherlight 12 ga. which is about 7 pounds. It has a strong, sharp kick, while my old (pump) 20 is hardly noticeable after firing the 12 at the range. Many factors go into determining how much recoil is too much. I weigh 230 pounds, and the 12, while stronger than the 20 doesn't bother me. My wife (1/2 my size) hates the 12 passionately.
