For a target rifle, the AR-10 is superior. For a battle rifle, the AR-10 is superior. Both platforms are superbly accurate. What hampers the HK is the horrendous trigger pull. Long, gritty and heavy. The AR-10 ergonomics are excellent and the rifle is well balanced. The HK is heavy... very front-end heavy. I prefer the HK's left hand charging handle to the AR's, but both allow you to maintain your firing hand grip. The AR wins in the ergonimcs department.
Sights are on par with each other. The AR-10 has a plethera of good scoping options while the HK is limited to either high sitting claw or clamshell mounts... or screw-clamp, low sitting mounts for the MP5 (which I prefer). The HK will eat just about any ammo you'll feed it. It's not picky. But the HK's fluted chamber essentially ruins brass, and the rifle tends to bounce empties off the side of the receiver damaging them even more. The delayed roller block action delivers a rather snappy recoil impluse that tends to be rather rough on optics. The fluted chamber spews powder residue into the action. It's dirty. I tend to spray Hoppes #9 into the receiver and hose all the carbon out.
Neca eos omnes. Deus suos agnoscet