Eh...here's the thing. If you think Hernangomez has any kind of potential, you trade O'Quinn now and its an easy decision, because you are selling high on his value.
This situation to me is clear. You have to bit the bullet on Noah's contract. Keep him around and play him as a third center or if you need him, but don't feel compelled to force him into minutes, because he's untradeable with his contract either way.
Then the question is what you think about Hernangomez. This isn't exactly a clear situation to fans, because we have seen so little of him this year. But remember Hernangomez is 23 and Kanter is 25. Kanter is way, way ahead of Hernangomez as a player and isn't much older. To me its clear that it makes sense for us to have Kanter for the long haul. This is a guy who is a leader, runs the floor, plays hard, plays hurt, is actually a pretty nifty scorer in the post. In the old NBA he'd be a real all star. In this NBA I think he still is a really valuable player because he does run the floor and he can be a matchup weapon. The best thing would be to give Hernangomez O'Quinn's minutes to develop behind him and then by the offseason, you've either brought his trade value up or you can see whether he can slot in for Kanter.