I think Kovu would win because the amount of jealousy on Nuka might set him to lose his mind and forget to defend and not do any kind of strategy or tactic but just attack which will make Nuka lose

Even though I prefer Nuka.

You can see that Kovu is a lot stronger build then Nuka. Also he is a lot more healthy and with his harsh training from Zira, he would pretty quickly in a fight.