If you go the shotgun route, I can highly recommend the Remington Managed Recoil 00 Buckshot. It is easy to shoot and groups very well out of my Remington 870.
A bit off topic for a handgun forum I realize.
If a handgun is an option I feel that it is a better choice than the shotgun. It is much simpler to move around with, and as Mike mentioned, a bit less obvious if you answer the door armed.