I wouldn't buy an AR anytime soon. They are way too expensive right now and impossible to get some parts.
Prices have come down quite a bit, and availability has gotten quite a bit better lately.
If it's truely for home defense, then I almost always say revolver in the handgun catagorie.
Why? I don't know of any organization out there that uses or recommends them over a quality semi auto.
Get a 500, add a pistol grip stock, ghost ring sights, and presto, the perfect home defense weapon (oh, and get rid of the strap Mossberg insists on putting on all their guns now. Retarded).
Why would anyone want a pistol grip on a shotgun? Completely destroys any advantage of a shotgun.
For rifle, I'd go lever action. Way more reliable than any AR or Mini14. Highly accurate at short ranges. And you can get it in a variety of calibers. You can get one in .357, then buy a .357 revolver. Now you have the perfect home defense combo. One type of ammo. Best of both worlds.
Since when? Are our soldiers and law enforcement agencies going out with unreliable weapons? I think not.
Second I see no advantage to using a pistol cartridge in a rifle configuration....might as well use the pistol. Not to mention that the lever action has many deficincies when it comes to reloading.