I definitely think think Koringhus would be Kellhus' intellectual superior, all things being equal.
However, Kellhus has quite a leg up on all the Dunyain because he has had so much more tutoring and breadth of experience, that I think a battle of wits/skills would see Kellhus as the victor.
Then again, Kellhus is totally insane, so who knows.
If they both stayed in Ishual their whole lives, I think Koringhus would have eventually surpassed Kellhus.
Might be a hint that Kellhus is not seeking the Absolute, since he hasn't offed himself. And, since it appears he regrets killing Moenghus, that they would have made a good team against defeating the Consult.