I own a Winchester SX3 with a 28" BBL in 12 gauge. I've owned it for 5 or so years at this point and have put it through its paces on a variety of upland and waterfowl species. When it was my go to gun, I put somewhere in the neighborhood of 800-1200 shells through it per year between hunting and clays. It's an absolutely fantastic gun. It's reliable, its durable, and it'll eat up everything from 2 3/4 #8 up to the heaviest turkey loads. I've used it in sub zero temps on the rivers of Montana as well as in 80 degree, humid climates in the south. It just runs.
Breakdown and cleaning of the gun is easy. To be honest, I think I may have cleaned 3-4 times over the span it was getting heavy use.
I do vividly remember the one malfunction I had while using the gun on a turkey hunt, but I'm fairly confident that the source of the issue was a bad primer on the shell.
The only gripe I had with the gun itself was the size of the trigger guard. I do most of my shotgun hunting on rivers and lakes in and around the rockies, so a larger trigger guard is preferred for shooting with a glove.
I picked up a Beretta A400 Extreme Plus last fall and have not looked back. I almost feel guilty turning my back on the SX3, but the Beretta has some really useful advantages to it that makes it my preferred auto-loader at this point.
I put some shells through a Benelli M2 20g last summer at a clay match, I was impressed with how it handled and shot, but thats the extent of my experience on it. I have a close friend who guides waterfowl on the Eastern shore and he absolutely swears by his M2 in 20g, despite him owning several SBE 3's and 2's. As for 12g vs 20g: the the modern 20g waterfowl loads (Hevi-Metal or Hevi-Shot) will knock down what you need it to out to about 40/50 yards. It's obviously great for upland, doves, quails, and clay shooting.
Hope that helps a bit.