20 ga is very good for people who are recoil sensative. Unfortunately, that means that the guns are usually lighter, so you loose some of the advantage there, but you still have most of it.

As far as weight per shell and muzzle flash, it will be negligable.

Two big draw-backs to 20ga.:

-It isn't as common. As a result, there is no were near the range of options for ammo as you find in 12ga, and nothing that I've ever seen in terms of reduced-lethality rounds (beanbags, etc). On that point, not much that can be done with a 20ga that can't be done with a 12-ga loaded with lower pressure loads. There are number of low-recoil loadings out there, but they are either sporting rounds (small bird shot at low speed, but not inconsiquential), or targetted to law enforcement (hard to find, expensive).

-20ga is a funny size, mechanically. I personally suggest NEVER owning both a 20ga and a 12ga, as a 20ga shell can slip down the barrel of a 12ga pretty easily. If you don't notice that a 20 has gotten mixed in, you could load it, have it go into the chamber, not go off, and you assume it is a dud. In a stressful situation, you load a fresh shell, and fire, blowing the barrel on the shotgun and probably making you a casuality.

Neither of those are enough to remove the 20ga from consideration, but are factors that need to be considered.