Well you gotta tell us what shells you want to shoot out of the 12 ga.

If you are talking about 3/4 ounce target loads then the 30-06 kicks a lot more than a 12 ga.

If, on the other hand, you are talking about a 3.5 inch 2 ounce maximum turkey load then the 30-06 compares about like a .22 rim fire. This load is down right painful to shoot. Even once.

A 30-06 firing a 180 grain bullet is going to have about 20 foot pounds of free recoiling energy out of an 8 pound rifle. A 12 ga shooting the above turkey load is going to have about 60 foot pounds of free recoiling energy. This is equivalent to a 416 Rigby. So you tell me which one kicks harder.