Actual IQ is comparable in my opinion. Just different "flavor". It is in ergonomics, physical buttons, speed and low light autofocus where the D5 is better (and battery life!). Z6 is, of course, more quiet and may be more precise with static and has an edge with eye af and the coverage of autofocus points. With manual focus lenses Z6 is way easier. Different machines for different purposes.
High iso images are very close, noise wise.