Eternal debate! I think we need two view opposition.
Khabib was mauling, charismatic never bled, always moving forward. His overall aura is superior to 99% of fighters.
I think the second belt is not that huge of a deal if the opponent is inferior (again, opposition, noone except fans remember Conor's second belt opponent but everyone remembers his win against Aldo).
But ! Islam ended Oliveira and Volk's huge winstreaks. If Islam beats JDM, he will again end a huge win streak (JDM 10 years undefeated). These achievements are superior. In comparaison, Khabib's biggest win is against Conor, and he had already lost to Nate Diaz.
Islam has accomplished much more in the UFC but, Khabib is undefeated. Maybe none of both is destined to be better than the other, but together, they are meant to make history