I dunno. Tatum has proved more at a much higher level (ECF), but Zion is light years ahead of where Tatum was as a freshman. Would be hard to choose either way.
I don't know that I'd trade Kyrie for him (considering he'd have signed an extension for that to be possible). Kyrie is much more of a position of need for us, especially considering Rozier is a goner most likely. He also puts us in a better spot to win, which is all we should be looking to do. Zion is a big unknown.