Islam overall is the more skilled and well rounded fighter.
Khabib however, is more effective and more dominant with his skill set, be it more limited. So that alone makes him the better fighter. Khabib applies his skill set more effectively along with his game plans and shows more dominance and success. If you take Khabib vs 5 opponents and the Islam vs the same 5, Khabib will be more dominant and perform better.
Islam however, will in the end have the great legacy because he kept on fighting and defending his belt and also fought better competition. Under the assumption he continues to win.