MAD DOG, most here on TFL have much more experience than I do, so take this with a grain of salt. However, I took a couple of Tactical Carbine / Shotgun classes from Gunsite recently, and our instructors in those classes made a strong argument in favor of using the AR-15 for home defense. This surprised me.
Their logic was that that AR-15's have plenty of knockdown power for BG's, you can obviously carry lots of ammunition and make quick reloads, and with 55 gr hollowpoint rounds they claim the penetration is less than 00 buckshot, and definitely less than a 9mm or .45ACP round (that argument surprised me especially). You can also mount tactical lights on an AR-15 just like a shotgun, if you choose that route.
Personally, I'm still a little goosey about the penetration issue, so I'll rely on my 870 with 7 1/2 birdshot followed up by #00 or #1 buckshot. But, my next choice would definitely be the AR. Someday I hope to test the penetration theories. Perhaps Shawn Dodson or one of our other ballistics guru's can chime in.
I still think an AR is one of the best resources I have in my gun safe.