I'm not a cop, but I play one on T.V.
haha
Seriously. I love the .40, I own 4 of them. I also love the 9mm, I own 2 of them. My readings and comparisons go somthing like this.
The 9mm is about 96% effective in the whole one shot stop thing (not starting a debate guys, just what I've been taught) but the 9mm lethality takes a little longer.
The .40 is about 98%, but the test shootings are much lower because compared to the 9mm...its a brand new round.
My dept. issues the .40 because it was a compromise between the rapid fire capability and accuracy of the 9mm meets the punch and pomp of a .45. Think about it, all you're giving up is .05...not much yet the round goes much faster.
The 9mm is still a very viable law enforcement round and is probably the most popular round used in law enforcement today. The .40 is catching up, however the .40 took a hit when the Dept. of Homeland Security decided to issue .357 Sigs to all of their FLEOS except the Border Patrol and the Coast Guard, who use the .40 and 9mm respectively. A few treasury dept. have also made the switch from 9mm to the 357 Sig. Essentially the 357 Sig is a real hot 9mm.
I believe that if I were going to defend my life in 1 or 2 shots, the .40 would be my pick, BUT, if I were going into someplace like Iraq where I may be forced to get into a knock down drag out take cover fire fight, then the capacity of a 9mm would attract me. If I were looking for a backup or hideout gun, the 9mm would be my first choice. If I were making a dynamic entry with a SWAT team, the .40 would be my choice. It really depends upon application. Lastly, if I wanted to shoot the eye out of a paper Osama Bin Laden target at 55ft. the 9mm would be my choice.
Hope this helps.