........ Also, anyone that says "9 mil doesnt have the knock down power of a .45!" ....Mr. Bad Guy isn't going to be concerned with what caliber he was just hit with...
X2. I carry .40 because my service weapon is a .40. In doing this I have extra rounds for both my primary and back-up weapons if one should be out of service. So now when I carry off duty, I just carry my back-up weapon which happens to be a .40.
That said. a .38, 9mm, or even a .22 will get the job done. If it comes down to it, the threat isn't going to look at the barrel and inspect it to see what size rounds you are firing at them. They'll see a gun, hear a bang, and with any luck feel a hit. I highly doubt anyone would stand there and say "it's okay, I'll still try to rob this guy, he's only shooting at me with a .22". Yes a .22 has less punch, but it is still lethal. Of course even to that extent I wouldn't recommend using that as a defense weapon.
IMHO, a .38 or 9mm would be a good choice. They are certainly sufficient, especially if you buy expansion point ammo. "Knock down power" is over rated. Yes a .45 has punch, but really even compared to a 9mm (.354 cal, which is .1" smaller) there is not going to have much of a difference.
A .45 will NOT knock someone over/back, despite what TV shows. It's simple physics, force equals mass times acceleration. What most of the smaller diameter rounds lack in size they tend to make up for with muzzle velocity. So there may be less mass, but increased acceleration can often times create a comparable force. A prime example is the .223 rifle round compared to a .22LR round. The .223 is only .003" bigger diameter (the size of an average human hair), but is considerably more effective. This is done by packing A LOT more powder behind the round, and it leaving at a much higher velocity. In the end it will come down to a very minimal difference in power between the .45 and 9mm (or anything in between). The biggest difference is how powerful it feels to the shooter. There's that and of course the seemingly important response of saying you've got a bigger weapon than the next guy. ha ha
Seriously though. If I was buying a weapon for concealed carry, and I wasn't trying to maintain the use of a single size round, I would look at a 9mm or .38. Both are easily sufficient for defense, they're cheaper to buy ammo for, they tend to have less recoil which lends to more accurate shooting, and you get more rounds in the same weapon/magazine size. For personal use I don't see any reason to NOT buy a 9mm or .38. Just my $.02 though, some people may think that was all a load of crap.