There is only one civilized solution.
Why a Shotgun
Power: A 12 gauge slug has as much energy at 100 yards as an AR15 does at the muzzle. Most of the urban area engagements will be at 50 yards or less. If there are threats greater than 150 yards, you should be fleeing/avoiding them, not attacking. A .223 isn't suitable for deer, and there have been multiple reports of troops needing to shoot targets 2-3 times to drop them with the AR. A shotgun, on the other hand, can take down 2 or MORE badguys with ONE shot! There have been reports of people killing two, and sometimes three perps with one shot of buck or shot.
Reliability: Pump shotguns are about as reliable as reliable can be, and can take untold abuse and still function and fire perfectly. The AR 15, on the other hand, has a horrid reputation for reliability and dependability. It defecates where it eats and fouls up easily, often jamming and malfunctioning, especially in dirty environments. In a combat and evasive situation, you may not always have the time to clean your weapon every other hour, like the AR requires.
Versatility: Shotgun can handle anything from varmints and pests to rioting mobs and out of space monsters by simply switching the choice of ammo. Birdshot, to buckshot, to slugs. Also, one can find 12 gauge ammo just about anywhere, even in the big city at places like Wal-Mart. .223 ammo, while fairly common, would not be common enough in an anti gun urban environment like New York City. Unless you expect to raid a police station or an X military base, do you really expect to find .223 ammo around the city? For one, in many cities it is illegal to own such a rifle, and #2, because of their scarcity due to anti gun laws and general liberal atmosphere, not many stores will offer .223 ammo due to lack of demand. However, by contrast many people in big cities who do, or can, own guns have shotguns.