exactly.
As posted above, shot placement is key. 3 solid hits with a 9mm is better than 10 scrapes and misses with a .50AE any day of the week.
First of all, make sure you pick a handgun that you WILL carry. If you don't have it with you, it doesn't matter what caliber it is, how small it is, or how good it shoots. Even if you find that the most power you can carry comfortably is a .380, that is still better than a 9mm at home.
Second, make sure that the handgun you choose is not too powerful for you to handle with regard to recoil. Many handguns such as Glock, M&P, 1911, PPS, etc have very similar frames, but multiple available calibers. If you can carry one frame comfortably, you likely have multiple calibers to choose from. Generally, most people will push towards the more powerful of the bunch. This is not always the best idea, as the larger calibers in a small frame may make your follow-up shots next to impossible to execute. This is dangerous for anyone in the area, not to mention yourself if you cant hit a target when you need to. The best way to find this out, is to try and get your hands on various models at a local range, and ask if you can try a few rounds to compare and contrast. My money is on the shooter letting you give their handguns a try.
Lastly, practice with whatever you have, a lot. If you end up with multiple handguns that you carry, make sure that you practice with all of them. You don't want to be so used to shooting a Glock that if you pick up a 1911, you forget about the safety!
For reference, I carry a Walther PPS in 9mm primarily, but also carry my Kimber Compact 4" CDP-2 .45ACP, as well as my Sig P229 in .40. The PPS is the easiest to carry as it is so thin, and the 9mm even with +p loads makes follow-up quite easy. I couldn't say the same for the .40 PPS. The Kimber is also very easy to carry, and a little extra size/weight make the .45ACP no problem at all. Lastly, the Sig is an absolutely excellent shooter, has a much higher capacity then the other two, and being able to convert to .357Sig is a major plus... but is a little more difficult to conceal being wider than the 1911, and especially the PPS. I tend to rotate shooting and carrying all of them, and those are the choices that I have settled on so far as carrying concealed, and generally alter what I carry based on what I am doing. If I am in light clothing, the PPS works best. If I am transporting high dollar goods with a "Jack Me" sign on the roof in the middle of winter, hands down the P229 is getting carried. If I am in a bad area where the sight of a large hole in the end of a 1911 would be most deterring, then the Kimber comes out.