I subscribe to the Gabe Saurez school of thought that the higher capacity of most nines compared to a 40 or 45 trumps the bigger size of them.
Honestly I say you are best off with what you feel more comfortable with. Yes the .40 is more of a man stopper, but if they are more comfortable with the 9mm then I say go with the 9mm. If they are a quicker shot with the 9mm because they are more comfortable with it, those extra seconds may save their life. Some of you say that it is better to have more of a man stopper, however I would much rather be shooting with something I know I am comfortable with. Some one on this site (can't remember who), has in their signature, "It is better to have well placed shots with a .22 than misses with a .44 magnum". I think this statement fits this question well.
In my opinion, two well placed shots to the chest with a 9mm can do just as much damage and stop a threat as a .40
I heard a quote one time by someone in the military. They were asked what they thought about the military switching to the 9mm round instead of the .45. The person who asked said it was stupid because the .45 was much more of a man stopper. The military person's response was "If I put two in your heart and one in your head...are you gonna know the difference?"
If you are comfortable with the .40, go with the .40. All I am saying is go with what you are comfortable with.
The 10mm is basically a "magnum" .40 S&W. The only difference is that the 10mm came before the .40 S&W. Historically with your special/magnum rounds, the special round is first then the magnum round is engineered later. In the case of the .38/.357 and .44 the late Elmer Keith along with Remington and S&W engineered the magnum rounds. There is talk of a .500 S&W special. The .500 S&W magnum came first engineered by Cor-Bon and S&W.
I'm sure there will be quite a bit of opinions on thins including other calibers. But I wanted to start a thread about this since I just got an email from a user with the following question:
I'll give my own opinion about them. Mainly, I think you need to shoot both and figure out what you like best. My favorite caliber is .40 S&W. At first all of my carry guns were .40. I think its a bigger round so I would hope it would stop the threat quicker than a 9mm. But you also may have less rounds when going with a .40.
Recently I decided to go with the 9mm Glock 27 as my carry gun. I haven't been getting to the range as much and I can just control the 9mm better in the subcompact. I also kind of just wanted a different caliber other than a .40. Now if I was practicing more like I should be I would probably have stuck with a .40 Glock 26.
Another thing is that it should be cheaper too shoot but I don't think the difference was that much so it was not really a deciding factor for me.
But, the gun I keep in my car and on my nightstand are .40. I am not concerned about concealing those so the larger guns.