Just a few more opinions to add to this discussion since my last post:
1) I agree with Sam, a less-lethal shotgun should not be the same as a "lethal" shotgun. It's too easy to end up in a situation (especially under stress) where an officer puts a load of buckshot into someone who only really qualified for a less lethal response. In my department the less lethal shotguns have fallen out of favor a bit, and we're now primarily fielding pepper ball and 40 mm less lethal weapons in lieu of the less lethal shotgun. Regardless, we still have less lethal shotguns available, and they have an orange forend/stock on them
2) One reason that shotguns might appear more popular in many agencies might have everything to do with agency politics, and little to do with effectiveness. In my department every officer is shotgun qualified in the academy. Rifle officers are required to apply for an additional 50 hour training course, and the waiting list for that course is usually 2-5 years. Additionally, to be eligible for the course you need to have a sign-off from your commander, and a demonstrated yearly pistol qualification average of better than 90% (we qualify quarterly). So, some officers opt for shotguns simply because it is a much easier option to choose.
3) Lethality is a moot point. If I shoot an attacker, and he can remain in the fight, but dies from his wounds 3 days later, that lethality has done nothing for me. Conversely, if I shoot an attacker and he immediately loses consciousness, the fact that he might survive his wound is also irrelevant. The goal is always simply to stop the fight. Admittedly the mechanisms of stopping a fight with a gun are often the same mechanisms that result in death, but the two factors are not inextricably linked. As such, determining the effectiveness of a defensive weapon on the basis of mortality can be a flawed way of looking at the situation.
4) A shotgun is a very good fight stopper, with proper ammunition and shot placement (yes, shot placement ABSOLUTELY matters with a shotgun at home defense distances). But, despite the effectiveness of this weapon, I've personally seen it fail to anchor people in the past. A rifle is also a very good fight stopper, again considering proper ammunition choices and good shot placement. But, like the shotgun, I've seen them fail to stop people in the past. Side note: using birdshot in a shotgun is foolish, IMO. We once had a domestic where the dirtbag boyfriend shot his girlfriend in the face with a 12 gauge as they stood in the living room together. She took the entire load to the face, and survived (though probably wishing she hadn't for a while). The light shot simply failed to penetrate sufficiently to kill her.
Anyway, in pure stopping power I'd probably give the edge to the shotgun when shooting against an unarmored person at close range with 00-Buck or slugs. The rifle gets an edge in stopping a person at longer distances, or potentially when the subject is wearing body armor.
Just remember, there is no magic bullet. Learn to run the gun you have, and learn its advantages and limitations.