Which one is more powerful, Kizaru or Rayleigh, in One Piece? Tell us why!

Before Rayleigh retired, Rayleigh must have been very powerful. Kizaru once said, if you are not fully prepared, even I will not be sure to beat you (referring to Rayleigh). But there won't be a big difference in strength between the two. Rayleigh will definitely spend a lot of effort to defeat Kizaru. Besides, Rayleigh is already old. He just needs to protect Luffy from Kizaru's attack for a while.