Volk has the better resume and is the better fighter, sure Khabib has looked more dominant but that is more due to his competition and style rather than his skill.
Volk has taken bigger punches and escaped tight submissions, I know he can do that, I don't know if Khabib can.
Khabib's biggest sub threat was Poirier, Khabib didn't have a threat on the feet because the strikers who could threaten him, couldn't keep the fight standing.
You have Volk beating a taller rangier elite striker standing up over 5 rounds, 3 times, now I know Khabib couldn't do that, Volk out-struck Holloway 199 to 124.
Then you have Volk moving up and appearing to be the biggest threat to the LW champ with some folks believing he beat the champ.
There really is no comparison in my mind.