I have to disagree with you on the power/range issue. Using the same size and weight pellet the 12 and 20 ga typically are shooting the same velocity (approx 1200 fps for lead and 1350-1550 for steel), therefore high school physics proves the force is the same per pellet. Therefore the range and power are the same. The difference is the 12 ga shell usually has more pellets. More pellets means better "chance" of hitting target at virtually any range. However a properly patterned 20 will out shoot a poorly patterned 12 ga every time.
The benefits of a 12 ga are:
- larger payload (more pellets per shell)
- more firearms to choose from
- larger variety of ammo to choose from
- usually lower cost
The benefits of a 20 ga are:
- less recoil
- smaller/lighter gun
- will match performance of many 12 ga loads
- smaller/lighter ammo to carry
Much of the above is correct, but there are so many things wrong with this post, where do I begin...
"Therefore the range and power are the same" - paper ballistics do NOT equal effective patterns on crossing birds in the field! Shot string is a real phenomenon that matters to skilled shooters. Range and pattern are definitely NOT the same, but a 20 ga. may be fully adequate for some situations depending on shells, choke and conditions....
"Less recoil" - depends on the gun weight and stock configuration, many 20 ga. guns are made much lighter than their 12 ga counterparts, and may actually recoil harder!
Another way of saying this is that recoil will be equal in equal weight guns, both throwing 1 oz of shot at 1200 fps....
The worst development to be marketed to shotgunners in a century is undoubtedly the 20 ga. 3" shell, which many would tell you (based on paper ballistics) equals a 12 ga. That is very far from correct, but those with shooting skills unequal to proving the difference continue to perpetuate that myth.
If you are interested in casual clay target shooting, either will work very well.
If you are interested in "take 'em as they come" waterfowling, only a 12 ga. is really practical.
If you are primarily interested in upland birds, most think the the 20 ga. is a little superior.
But for shooting gophers with a knoxx stock , either will do!