Quick and accurate wins the fight no matter the round and don't let anyone tell you 9mm is underpowered, it is as deadly as any handgun round provided you use a modern loading and place them in the vital areas. There is no such thing as "knock down power" with handguns, they make little holes, some fractions of an inch larger than others. Use what you are most proficient with and you will be much better off than just choosing the the most powerful first round defense.
By the way, here are some 9mm numbers....
http://www.ballisticsbytheinch.com/9mmluger.html