I think the argument should be if you had to pick one which would it be. Everyone wants to have both. The more guns you have the better off you'll be. I have both a r15 and 12ga and love the capacity and accuracy of my ar. I want to have both ( and a grenade launcher and an uzi and an rpg) However, the 12ga is more versatile. You can hunt different game with it and I think the ammo would be more easily available. If your picking off zeds from greater than 100 yards then your not in great danger.
The low capacity of common shotguns has been dealt with by the SAIGA Shotgun by IZHMASH. Izhmash is the same company that produces the AK rifles for the Russian military. There is a problem though. The BATFE may ban them unfer an administrative ruling. NOT GOOD. I'm sure someone in the US will build one though. There is a market for it.