My choice is a standard E2 configured AR, . . . 30 rd mag, . . .
Reason: my .223's go out 1 at a time, . . . whereas your 00 buck goes out in clusters. I can slightly move the impact zone for my .223, . . . where ever you pulled the trigger on the load, . . . that is where they land.
Inside a house, . . . you will not get effective 12 gauge expansion of the load until you get down to a 10 or 12 inch barrel (very much illegal by the way) but the AR gives you the ability to "spread the wealth" so to speak.
There will always be the "knockdown power" argument to counter my reasoning, but if you don't hit em, . . . knockdown power is a moot commodity. Additionally, . . . reloading a shotgun is a time consuming, action rich adventure in a high adrenaline event, . . . reloading an AR is a piece of cake, . . . push the button, . . . I've got 30 more.
AND, . . . if I really wanted to go "HI TECH", . . . I saw I believe 60 round mags that are almost the same size as 30's, just fatter in the bottom, . . . if you cannot stop a BG with 30 or 60 rds of .223, . . . a 12 gauge certainly will not be any better.
Just old codger thoughts, . . . mostly influenced by the dead bodies of our MP's on the streets of Saigon, Hue, and other places, . . . when the powers that were then forced them to use shotguns instead of rifles. It took a number of bodies, . . . but they finally listened, . . . gave the shotguns to the infantry who could use em.
May God bless,
Dwight