It's close but I take Rantanen and the 1.600 in cap space
I think we will need to see how they fare next season. Rantanen plays on the better team and has arguably the best or 2nd best player in hockey on his line, but Marner's linemates are no slouches either. I would expect if the players were even, that we would see Rantanen slightly ahead due to situation, but Marner created quite a gap this year.
Small sample size, though. Let's see what 2021 brings.
Either way, I think there is so little separating them that it can swing either way on any given season. I'm glad I cheer for a team that has one of them, no matter which one it is.