Suppose, if that prof were in columbia, then I think you will definitely choose columbia, but he is in nyu, then you do not know which one to pick, indicating that you know columbia is better than nyu. So, I would say which one you pick depends on the weight of that prof in your decision.
i do not think there will be any open in columbia even if lz declined it, since as lz said, they sent 8 offers at first round and I think the number they really need won't be larger than 3.