Read this and it will tell you everything you need to know. It's not velocity, that's part of the equation, but so is mass, and shot placement. This article states that the .40 is the best, but the author is adamant that the 1 factor is placement!!
My favorite quote in the whole article: I reminded them of my 1st commandment of gun fighting: "No intelligent man or woman should ever take a pistol to a gun fight with any degree of enthusiasm."
http://www.neiassociates.org/caliber.htm