theres really several more factors to be figured in... besides liking the cool black rifle the cops and military use.
1st... if its a true defensive firearm, why 223/5.56... I want allot more range. I prefer 308/7.62, its got more range and more knockdown. I want to be able to take out zombies way before they get close to my perimeter. 308 also has more close quarters knockdown. Pick the right one like a Springfield, HK, or Fal... and you have a pretty good all around rifle with about the same capacity as an AR.
Now for a SHTF firearm... I like an AR or a Ruger 10/22. The AR is good all around, and the "zombies" that you might encounter will probably be carrying an AR as well... so ammo, mags, and parts will be plentiful. The Ruger 10/22 would be good for taking game... or if you dont have an AR... it might help you obtain one. You can carry 100 times more 22lr then you can anything else.
Shotgun is fine too in a defensive situation where over penetration of a rifle round may be an issue.
So it really depends on what you want, can handle, and afford.