9mm is perfectly adequate for defense as long as you use good hollow point ammo. It is also less expensive to shoot for practice and recoils less than .40.
A lightweight polymer-framed .40 auto is not one of the better guns to learn to shoot with, at least not if you want to shoot really proficiently. A well-placed 9mm will stop a bad guy much faster than a poorly-placed .40.
(The .40 fans will soon come out of the woodwork to say the 9mm is a peashooter and the .40 is the right hand of God Himself, designed to smite the enemy from on high. This is expected on the internet and generally to be ignored.)
As I am the leading advocate of Glocks on this forum, I will tell you that strongly prefer them to USPs. Glocks have the advantage of low bore axis (less muzzle rise) and a short/firm trigger reset. Some people do prefer the USP, however, and if you are one of them, bear in mind that you can get the USP in 9mm.