Shotgun v. AR? Well, what about continuity of training? I run a semi-auto, mag fed pistol at work as my primary defense. Working a slide-action shotgun is a whole nuther animal. This means two types of training, two types of motor function.
What about varying users? Husband, wife, children, parents, aunts, uncles, friends.....just who might be called upon to use this weapon to protect themselves or another? Can my mom, for example, use that 870? More importantly, can she use it effectively, including reloading, and not worry about short-stroking the slide? Will my nephew be able to employ the Mossy 590 and save my bacon when that rabid dog is circling me?
After thinking about the types of people that might need to use one of my carbines, and nothing else, I had to take factors than penetration into account. My choice, after doing all this question asking, was to go with an AR. I didn't come to this conclusing immediately.
Like some, I thought to keep things as simple as possible and chose the Marlin Camp Carbine M45 for home defense. What better for a HD carbine than something chambered for my pistol caliber and using my pistol magazines? Bad idea!
While the longer bbl give ample time for a +P powdercharge to achieve full burn, thus increasing the power in the bullet, the platform sucked big ones. Marlin really dropped the ball with this carbine, and should be ashamed.
After that experience, I opted for the AR in a fairly simply M4-style. The flat-top receiver will allow me to mount an optical sight should I ever decide to, but I probably won't. The railed forend, just recently purchased, drops straight in, no mods/gunsmiths, and gives me a place to mount a flashlight so I can ID perps in the darkened interior. The collapsing stock allows me to shorten the LOP when plinking in heavy winter clothes, or to train my little nephew on basic marksmanship, or for some lady that might like to shoot.
Nothing fancy, no frills to go wrong, and a minimum invested.
For in-house ammo, my barrel has a 1/7" twist. Because this is rather fast, I chose to load up the first mag with UMC 45g JHP ammo. From what I gather, the fast twist of the barrel will make this round very unstable after contact, with a max of around 8" of penetration in gelatin. The chances of it being a great danger after penetrating a wall, esp the walls in this old farmhouse, are slim. The backup mag on the buttstock is loaded with 55g FMJ, just in case I need penetration or harder hitting power for a rabid dog or barrier penetration.
With a good two-point sling (I'm trying the VTAC) and a light source, you have a short, light, maneuverable carbine that is good for plinking, training, and defense. Nothing fancy. No frills to go wrong. And a minimum invested.
Just my thoughts, though. Your mileage may vary.