If I were going to buy a defensive shotgun I would buy a Vangcomp 870 or a Vangcomp Mossberg 590. I would probably buy the Mossberg right now as he has a great special running on the 590A1 right now. I was also less than impressed with some aspects of two Vangcomp'd 870's that I shot next to some time ago, although I do not blame Vang for this. The patterns were INCREDIBLE out to 25 yards, but the 870's were both much slower than my plain jane Mossberg 500 (stiff nechanisims, bulky forearm, and a safety placement that is slow) and had fairly consistent problems where I had none. Look for the Vangcomp website, you would be hard pressed to find a tighter shooting defensive shotgun. One 870 was shooting less than 6" with 00 Federal buck at 25 yards!
I have a 226, and a 220 Sig, which I love and hate. I love the way they feel and point, but I cannot get a consistent first shot with the double action first pull. I am still working on it, and the guns are reliable enough, but I pull that first shot low no matter what I do.
The AR is, well an AR. Most fun and comfortable high power rifle to shoot in my opinion.
I say buy them all!
If you cannot do that, you have to get a bit more specific about what you want to accomplish with the purchase.
Ray