FWIW, dogs should primarily be used as early warning devices and visual intimidation for those with ill intent. If you rely on a dog for physical protection, you've already failed. Especially if you care for and have a bond with the animal, which is pretty much essential if they're gonna protect you. If the dog is injured in a physical confrontation with a human or animal attacker, and they will be, the dog will then become a liability. Dogs require a fair amount of maintenance even in good times, and in a grid down scenario maintainig the health of you and your animals will be priority #1. In the proper context, dogs serve a limited, helpful role. Having said that, medium sized mut is the way to go. Of course if you want a real working animal that requires no maintenance, adopt a pack of coyotes. You definitely won't have any rodent problems. Or cat problems for that matter.