I assume Mossberg is doing their 20 gauge on their smaller frame gun as Remington is with their 20 gauge Tac-14. I find the smaller frame shotguns are a delight to handle in the house, just as they are in the hunting field. I have an 18" 20 gauge 870 now as one of my home defense guns, and am very happy with it in that role. 20 gauge #3, #2, or #1 buckshot will all penetrate sufficiently and launch at similar velocity to 12 gauge loads. 328 grain, .61 caliber, 20 gauge slugs will also work just fine for home or yard defense.
If I was going back into the field as a cop, (not likely at my age), I would prefer the 12 gauge I carried for it's increased payload at longer ranges. But for an uninvited guest at 3 in the morning, I'm perfectly fine with 9 pellets of #1 buckshot at 1300+ fps across my living room or yard.