Ak-47 or AR-15?

Something that you are capable of using and hit something with the first shot. Like a short barrel shotgun.

The 55 gr .223 rounds is better than the 7.62x39mm round because it is less likely to overpenetrate and go through as many walls if you miss, all do to it's tendency to tumble and fragment when it hits a soft barrier. Therefor the AR-15 is a better choice. Also AR-15s, are a little more ergoomic and easier to mount red dots and lights.

Either would be fine. I prefer an AR-15 because I spent 6 years in the Infantry and they are second nature to me. I own a Colt LE6920.

Semi-automatic 9mm pistol with 17 rounds. Requires 1 hand for operation, easy to maneuver, you can carry someone to safety and still shoot, etc. Plus you can keep it with you while you're at home. That AR is stuck in a closet or under your bed when you need it.

It's more you than the gun, AR's are very modular and easily customizable to specific tasks but there is a large aftermarket for AK accessories as well. It really just comes down to personal preference, the 223/556 cartridge may actually be better as it's potential for over penetration should be less than a 7.62 round.

Neither are what I consider an ideal close quarters defense gun. I'd much rather have a compact shotgun. If I had to choose one of your choices, I'd pick the AR only because i'm not a big fan of AK's.

Get the AR. Chances are, You will never use it to defend anything but its alot more fun.

both over penetrate neighbouring walls

Get a rubber fork. The mere fact that you asked this question indicates you're unsafe with either one.

Super soaker.