Currently, I carry 40 rounds of .40 on me, before that, 49 rounds of .40, before that, 37 rounds of .45, and before that, 46 rounds of 9mm. Grant you, the reason for so many rounds is because I always carry 2 spare magazines, not because I think I'll ever be in a situation where I'll need that many bullets (although that's always a possibility.)
Yes, I do prefer higher capacity firearms, but that's a personal preference, which stems from the AWB of 1994-2004. Consider it "thumbing my nose" at those who say that civilians should be limited in how many bullets they should have available to them.
How many rounds you carry in your firearm isn't as important as carrying spares I believe. Whether you go with 9mm or .40 you've got a good self defense round, and you can't go wrong with either with today's SD rounds. Buy whatever you're most comfortable shooting, just make sure you're carrying at least one spare mag.