While I own a Gen2 G19, I'm more of a .40/.45 guy so what I can compare is my Gen3 G23 made in 2004 and my USP-40C made in 1996. While the Glock is good in that it doesn't spit brass into my face ala Gen4 style and is combat accurate, the USP-40C is still the better gun. In both cases, reliability has been 100% with both guns so I don't even consider this a factor from these two makers. However, it's better fit, finish and accuracy are what sets it above the Glock.
Yes, the Glock is plenty "combat accurate" but the H&K is "combat accurate +P" so-to-speak. The LEM trigger is excellent and if you're worried about a mechanical safety or don't want the DA/SA trigger pull, the LEM is perfect for you. The LEM trigger also allows you to do something no Glock is able to do and that is give you a second strike capability. Might be a factor in your decision based on your training or maybe not but it's an option the Glock can't match.
Now the Glock with it's lower bore axis does allow faster split times between shots. When I was testing several .45 ACP handguns years ago (never got around to testing the .40 S&W handguns) my G21 was turning in faster split times than my USP-45F. However, those times were about .002-.003 faster so is it really all that much faster to make a real difference? I don't think so.
One other thing I discovered is that like the .40 caliber models, the .45 ACP Glock and USP were similar in accuracy in that the USP had the edge. Part of my testing battery was to run an El Presidente course of fire twice with a reload for a total of 12 rounds fired then scored with a Virgina count. The Glock 21 did turn in slightly faster times but the better accuracy of the USP-45F turned in the better overall scrore.
Honestly, I don't think you are going to go wrong with either choice. The HK is more modular in you can set it up the way you want it but the Glock is going to allow you more bells and whistles in the form of aftermarket accessories. If you have the budget for the higher priced H&K and want the better finished pistol with better accuracy, I say go for it. If however, you are more limited in what you can spend, the pre-gen4 Glock isn't a bad choice either IMHO. Then again, may as well do what I did and buy both and enjoy them equally.