As many have stated, too short is too short. My experience with the Mk18s (10.5") was less than exceptional - they required a lot more maintenance and a lot more lube than a standard M4. My carbine would barely run through a 200-round range session without reliability issues - and had to be cleaned spotlessly between sessions.
Contrarily, our agency-issued M4s are the definition of abused and neglected, yet I've yet to see one hiccup that wasn't directly related to ammunition.
Unfortunately, I have limited experience with the "tween" sizes - 11.5 etc...
ETA: I whole-heartedly agree that the shorter carbines are significantly easier to maneuver in/around vehicles and in tight quarters (such as indoors), I just don't know that the juice is worth the squeeze for a general patrol rifle. I think an M4 pattern 14.5 or 16" semi-auto makes perfect sense as a general issue should weapon; shorter barrels probably only needed for specialty teams.
And unless I were funded directly out of President Trump's personal accounts, I wouldn't even LOOK at HK rifles. I'd take a Colt any day; there are PLENTY of "civilian" 16" carbines that will do EVERYTHING a patrol rifle needs to do...