Well, interesting question that has been asked over and over and over, but is still a good question. My daily carry most days holds nine rounds of 9x23, and my two spare mags each contain nine more. And then, I carry a spare gun. The idea that, "If it takes more than (fill in the blank) shots, I'm not going to make it anyway," amounts to preparation to die, not to survive, and since the magazine is the part most likely to fail in a semiauto, you should carry at least one spare mag, regardless of how many rounds are in the gun.
Sure, statistics say you're not likely to need more than three rounds to solve Problem One. However, statistics also say you're not likely to have to shoot anyone anyway, and it's not going to be a statistic that is trying to kill you.
Eight .45s vs. 13 9x19s is less important than how well you shoot one or the other, and which you're most likely to ALWAYS carry. I find that I shoot single-stack guns a little better, since I have small hands. Also, with a single-stack, both the gun butt and spare mag are easier to conceal, so those factors make it more likely that I'll be carrying a single-stack. In terms of the warm, fuzzy feeling that goes with being adequately armed, I'm happy with either choice.
And, I like blondes, brunettes AND redheads...