It is my belief that the 20ga shotgun would be a lot more popular with Law Enforcement were it not for the 12 gauge's versatility with Less Lethal rounds and breaching rounds and so forth. Honestly, to really wring effectiveness out of an 18" barreled twelve-bore fowling piece loaded with full-steam buck loads, you need to be a physically strong, well-trained shooter, or a liar. (Believe me, I've seen some of y'all shoot. You'd do better with lighter loads. I know I do.) This is why 99% of the police departments in America have gone to reduced-recoil (usually called "Tactical", in deference to the manhood of their users) 8-pellet loads.
Remington's 20ga 870, made on a smaller receiver than their 12ga model, is what I'd pick if I were going to go buy a new home defense shotgun. It's slimmer, lighter, and all-around more handy than the 12ga model, follow-up shots are easier, it won't beat you senseless when training (you do train, right? I mean other than with a keyboard,) and at across-the-room distances with buck or slug it will cancel a birth certificate every bit as effectively as its larger sibling. And I need Less-Lethal or breaching rounds like I need a hole in my head. Many other shooters, even those of the big, manly, experienced variety, have expressed similar sentiments to me.
Plus, as everyone in the blogosphere has now mentioned, it is available in terminally kewt (get it?) pink. I want one. If you tell anybody, I will come to your house and shoot you in the kneecap.