I have a USP 9 full size & Sig 226, so not .40 but still a comparison betwee full size offerings from each manufacturer. Both good weapons, so you won't have any QC issues but there are clear differences between the two and if I could only have one it would be the USP, no doubt. The HK has a much better trigger, it feels lighter and quicker. I found the HK factory sights superior to the night sights that came with my Sig, but have since bought trijicon sights for both, so no issues there. The slide release is harder to reach on the HK and awkaward to operate the first few times. I'm accustomed to it now, but it's a bit weird at first and an area where the Sig will feel more natural. I also bought my USP when HK was still doing that ridiculous proprietary rail on the bottom instead of standardizing it with pretty much everyone else in the industry, so attachments are more expensive. That said, if you buy a newer HK design like the P30L (which I'm looking at now, but haven't bought yet) the rails are standard. No issues with the Sig WRT rail interface.
Both are easy to take down. Both point and balance well, although the clear winner in feel and draw is the HK to me. The things that put the USP over the Sig IMO are the trigger, the weight and balance, I like having a safety over just a decode like the Sig, the factory sights and I also much prefer the mag release. The USP literally shoots the mags out and gives a very audible click when they are fully inserted. The Sig is fine, but nothing special.
Hope that comparison helps. Note that all things HK (like new mags) will be more expensive. Worth it in my book.