Shipwreck described them really well. The trigger on the QA is outstanding - only about 3/8" travel @8# - every time. The HK (I have P2000) LEM is looong like a DA/SA - it just is not sloppy, as there is a bit of tension along the entire stroke. It is annoying at 1st but one can get used to it. I had shot a 1911 for a long time, then the QA and am now pushing the HK 'cause it feels so good in the hand.
Springs are available that can be mixed and matched in the HK to lower the standard 8# LEM trigger pull.
The QA fit is 1st rate - and the P2000 is maybe 1 notch better for CCW because it has the 3.5 bbl. The USP probably wouldn't feel as perfect balance-wise but is still OK.
The P2000 handles recoil better then the QA. It has a polymer bushing in the recoil assembly that seems to work, as the P99 is a bit sharper. I am not sure if the USP has this same feature.
The P99 has an unobtrusive decocker button which is a really nice feature; this is now available in some versions of the LEM. P2000 has ambi side release not sure about USP. HK has steel sights, Walther has plastic - adjustable rear and switchabe fronts included.
Both are GREAT guns and if cost matters - you would do well with the Walther. I would recommend the P2000 vs the USP for CCW - changable backstraps like the P99, better feel, recoil bushing.
Both my HK and P99 are near 1000 PERFECT rounds - NO problems in either weapon.