It depends what you're goingto do with it. You mention "knock down power" so I assume you're thinking about personal defense. If so, an autoloading pistol is a poor choice for a first handgun. While they're more reliable that they used to be, unless you possess a significant proficiency at clearing drills, in the dark, while being accosted or fired upon, that failure-to-feed might cost you your life. A revolver is a much better choice. 38 special with a +P load is the ballistic equivalent to a .40 S&W and a .357 Magnum is going to have a distinct margin over the 45ACP. Plus, a pistol you don't have with you isn't really of much use. If you're going to carry, the HK45/USP is a large, cumbersome piece designed to be a military sidearm, not a conceald carry weapon. If it's not comfortable, it will likely get left at home.
I tell EVERYONE this same thing about a first handgun. Make the commitment to become proficient in its use. Take some lessons. Most CCW instructors offer advanced training. Then shoot often. If you're not shooting at least 3 or 4 times a month, you have no business carrying a weapon.