9mm is a great round, as many before me have posted. One of my favorite things about the 9mm cartridge is that most 9mm guns are double stack (unless your looking at tiny pocket pistols), which means that you will have anywhere from 13-19 rounds in a magazine. Most .45ACP guns are limited to 7-8 rounds (there are a few double stack .45's though) If the SHTF the first gun I would grab would be my CZ-75BD, which has a 16+1 capacity.
9mm and .45ACP are relatively close in ballistics tests, so I would rather have double the 9mm rounds if things went sour. I absolutely LOVE .45ACP, but if the world was going to end tomorrow I would reach for my 9mm, easy as heck to shoot, great capacity, and the most common handgun round in the world.
---Sig P220 Match Elite/CZ-75BD/Ruger SR1911/Sig Mosquito/Savage Axis---