Probably sales.
I think legend status transcends any of the three though. I'd say Michael Jackson is a legend because of his performances, I'd say Madonna is because of her boundary-pushing. I don't think you can be a legend just because you won 26 Grammys (Allison Krauss) or sold 23M of an album (Norah Jones) or produced five #1s from a single album (
). You need something to set yourself apart.