Deleted. I need to find out how to make a poll.
I’m going to answer anyway. They were very different dancers, but I’m going to give my nod to Kelly as the more versatile – he could do everything dancing that Astaire could do and much more besides. He made a more convincing love interest too (as Dorothy Parker said of Astaire, Ginger Rogers “gave him sex,” but Kelly didn’t need anyone to do that). Astaire was a marginally better singer, but you didn’t ask that.
I’ve asked for the thread to be closed, pending receiving instructions on how to post a poll. (It was easy before Discourse.)