Uber Immortal is a non factor here. The only thing he had going for him in his fight with the Spartans was damage soak and enhanced strength (which was inconsistent, considering Dilios blocked one of his blows, even when the UI had downward momentum behind the strike). He was slow and displayed zero impressive skills as a fighter.
Khan and Bane both have durability and strength, but are much more skilled, and Khan was somewhat better than Bane in all those categories.
So Khan should actually take this fight.