RE: 12 gauge to 20 gauge....your opinion
Depends on what you are talking about. If you are talking about a dedicated slug gun with a rifled barrel and modern sabot bullets then yes they are pretty good. More than enough to kill deer at 100 or so yards.
Are they more accurate? Well that is a subjective thing, I have seen some pretty accurate 12 gauges. However the twist rates in the newer 20 gauges seem to be better matched to the ammo they are using so your chances of finding a load your gun likes are probably better. The new 20 ga ammo is right on par with using an inline muzzle loader, maybe a bit better. They shoot the same bullets at simular velocities with the same barrel twist rates.
Now if you are talking smoothbore with rifles slugs I would stick with a 12 ga. The only thing a 20 ga will offer you is a bit less recoil. However you will have less power, a smaller slug and less range most likely. Accuracy might be better, then again it might be worse.
Just my opinions anyway.
Paul