Oh man... I love these discussions.
I own and carry everythng from .380 to .45 and I never feel "under" armed no matter what I carry. I love my TCP, M&P 9c, Taurus .38 2", and my 1911. Each has a place in a carry layout, depending on what I'm wearing, where I'm going, and what I "may" encounter.
None of the above weapons provide a one-shot stop due to caliber! None! i can stop a fight faster with a well placed shot to the CNS with a .22 than a poorly placed shot to the arm with a .50AE. Now, here's the problem....
Every caliber is a compromise. All of 'em. With the smaller calibers you are sacrificing mass on target which can translate into disorientation and shock value to your target. Remember, if it's not a CNS shot, forget about "stopping power & killing power". With the larger calibers you're sacrificing the ability to make several follow-up shots if needed(They'll be needed, without a doubt).
I'm not going into ALL the aspects of caliber choice and the reasons why, not enough time. But, I will tell you that a CCW piece is for use within very close encounters, like real close! So, with this as your main criteria... caliber becomes a little less important. A few .380s to the chest at 2ft will drop a human as fast as a few .45s to the chest at 2ft. Don't believe me, oh well. Keep in mind, you WILL fire more than once!!!!! You will!!!!! So having the ability to make enough shots really makes the difference. Not all of your shots are going to hit center mass, they're not. So, would you rather have 6-8 .45 chances? Or 12-17 9mm chances to drop a threat?
Me, I carry what's going to be the most concealable, and the most comfortable for me on that given day.
Practice, practice, practice!!!