The 20 ga really doesn't have significantly less recoil. A gas operated shotgun will have a softer recoil than the inertia(recoil) guns. The Mossberg 930 is an up and coming shotgun, mainly due to it's price. Don't buy a "tactical" model. They just don't do well in the game.
You mean door breacher muzzle brakes don't help?
Can you guys give me more info on your thoughts on shotguns? I'd been considering several models - the Beretta Tx4, the FN SLP 22", or maybe a Benelli M2. The Beretta only comes in 18.5 and has a limited capacity although I've heard that Benelli M2 extensions will fit - and it's based on their field guns so perhaps it takes their barrels as well. The SLP seems like a nice all-around gun. The M2 is expensive and hard to find in the proper configuration.
As for Mossberg, it feels cheap and it has that ugly breacher on it.