I like 40's too. Best thing about it is you get the ammo capacity of a 9, but its not a 9!
When you have a short barrel such as those found on a handgun, nothing beats caliber. I am partial to a .45 myself, though a .40 for its larger ammo capacity has a certain attraction.
9mm - smallest rd I would ever rely on for a handgun, and quite frankly, I wouldn't buy one unless it came with a designation of MP5, and was about 140gr in weight or better. 115gr is about as useless a rd as you can get. Ask those cops that shot one guy 32 times before they managed to kill him over quite a long period of time. You won't have that problem with a 40.
Just my 2 cents. My caliber idea doesn't hold true for rifles though since you have a longer barrel with translates into a lot longer acceleration time, usually resulting in close to 3 x the velocity.
-S