I am of the opinion that the 20ga is a better choice than 12ga for most people. A 20 will give you more than enough "oomph" to get the job done, with alot less recoil.
The difference of a little lighter shot charge is more than made up for by the reduced recoil, controllability, and confidence that is inpired by being able to put more shells thru it in practice situations.
People always seem to get pushed into getting the biggest hammer, when most of the time a smaller hammer and a bit of skill will fit the needs better.
NRA Life Member, SAF contributor.