I live smack dab in the middle of a major (but not mega) city because it gives many more economic choices and options than any small community would. I look at the past few weeks, and in part due to the international nature of this city (and most West Coast port cities), and the fact that I have mastered "Business English" with an understanding of the thought process of various foreign cultures, only about 20% of my clients are American by birth.

Now this may vary city by city, but reality is that survival is still possible in the cities, but it depends a lot on how the were originally designed and laid out. Most in the southwest would quickly become non-inhabitable due to the lack of water. Many in the north would have problems without centralized energy distribution hubs providing for heat. Now in a middling sort of thing though, we look at the big freeze, snowed in winters that have happened, in the cities, the very poor can get to warming centers. Schools, churches, other public places set up and heated overnight. In the country, you lose power and heat, then read in the newspaper about how some isolated old folks died overnight when their relatives could not get over to check up on them, or someone got hurt loading firewood, fell asleep in the snow instead of making it back to the house, and did not get found until the snowdrifts thawed in the spring.

Then you have the roving criminals. A real problem in isolated places where sure, you may be armed, but the bandits can get clever about that too, and don't think you can always outsmart a pack of murderers when you have regular country chores to deal with. Now if SHTF and you have an organized neighborhood, that can be a different story. Maybe police response times might go up, but with the right street politics, your "buddy with a 12 guage" response times can be cut down to seconds because he lives just two houses down, and when trouble is in the air he keeps the hardware handy.