As a Bostonian I would never be allowed a permit to carry within the city unless I routed my request chit through god himself or Ted Kennedy. Not believing in god that leaves me with Ted and I don't travel in those circles.
I wonder why are you carrying a weapon anyway? Self defense?
Carrying a weapon for combat and carrying one for the street are two different things. A .45 is definately my choice if I need to stop somebody or if i need maximum effectiveness from each round. In a self defense situtation I would likely go with a double action 9mm. Double action is taken for granted nowadays. You really don't wanna have to carry a weapon at half cock or cock the hammer in a split second situation...
I would imagine a self defense situation to be close in and very fast. A 9mm is easier to conceal and likely easier to draw and fire. I can get a service M9 from hip to hurt in a blink of an eye, but I'm military and I train to do this. 9mm is also more controlable. I would like to have as much control over a weapon in close quarters as I can.
As for stopping power I will say this: If you hit somebody close in with a 9mm and they do not fall you have other problems. If you give them the whole magazine and they do not fall you have other problems. You need to withdraw at maximum speed and let SWAT team handle them with assault rifles.
If you are at range you probably don't need to get involved. There is no need to draw a weapon and engage in a situation unless your chest says US Army and you are patroling the streets of Iraq. First you need to call the police. Second, you stand a real chance of being shot by the police when they show up and assume you are one of the trouble makers.
Lastly there is overpenetration, as mentioned before. You want to hit what you are shooting at and nothing else. Maybe it was E i forget who it was that said it but exotic rounds will make you look like a maniac in court. Second you don't want to shoot through your target, the wall, and the bus full of orphans on the other side.
Oh and as E said in the get go, go to the range and decide what you want to shoot first.
In the case of Zombies just aim for the brain. Caliber really doesn't make much difference as long as you can destroy the brain...
Just my two cents, I'll be quiet now.