he probly would have went with simba because he looks up to his dad but on the other hand it depends on how much he may have known the relationship between his sister kiara and kovu
He would probably be on either Nala's or Kiara's side, because he knows that Kovu and Kiara love each other, even though Simba doesn't care. He would probably tell Kiara to find Kovu while he fights Simba to the death. On the other hand, Nala and Kion would probably confront Simba before Kion and Simba fight to the death. But, one-on-one, Kion would be mostly on Kiara's side than Nala's. But I'm sure Nala would try to stop Simba as well.
i think kion would have taken kiaras side not simbas because he is her sister and i think he can give kovu a second chance and the rest od ziras pride but not zira her self but i think he would give kovu a second chance because he knows that kiara loves him just like he love rani just like uf she dide something bad kiara would give her a second chance because she knows that kion loves her.