I recently purchased a Saiga-20. This shotgun is a twofer. With standard 2 3/4" shells, and set on the proper gas setting, it is a lightweight, light recoiling shotgun for small framed, or recoil sensitive individuals. With 3" magnums, and the proper gas setting, it is a lightweight, hard shooting shotgun for those who like recoil.
One advantage to the 20 gauge 3" magnum is that Federal makes a #2 buck load. I prefer the larger buck sizes, but this is apparently as good as it gets in a factory load. (I vaguely remember an article from the '70s where I thought they handloaded #1 buck in a 20 gauge. I would appreciate any comments on this area.)
The Saiga-20 is cheap, reliable, rugged, and definitely (f)ugly. The major drawback is the increased price of the ammunition.
When it comes to longer range, the 12 is definitely superior. Close up, I don't think any one is going to be able to tell the difference.