"Take down" power
Remember a .40 is only ONEmm larger in diameter (uses a 10mm bullet) than a 9mm. You think if you shoot a guy with a 9mm he will keep running thinking, "whew if that was ONEmm larger I would of been screwed"
You get 18rds of ammo in a G17. Not to mention the disadvantage of follow-up shots from the .40 because of its snappy recoil.
But to save the sake of beating a dead horse, we all have personal preferences so guys looking into different calibers, choose what makes you happy. But what ever you get, be trained and proficient with it. A small hit is better than a big miss.