The H&K USP is a great pistol if you like how it feels and points in your hand. I used to own a USP 45 full size, but found that I just didn't like the large size of the pistol, and also that I preferred a simpler manual safety free system.

And the .40 S&W is an outstanding cartridge for a defensive handgun. My personal preference for ammo has always been Federal Hydra-Shoks. They have a good real world performance record, and I've never had anything less than 100% feed and function reliability (perhaps the most important factor for a defensive pistol). Just make sure you run at least 100 rounds of whatever ammo you want to use for defense through it to check reliability, especially rapid fire, 'cause that's what you'll be doing if the s**t ever hits your fan.

Mike