Uh, tie. Both are admiral level and are portrayed as the first mates of their respective crew.
If anything, I'd lean more towards Marco. They both had their own quarrel with Kizaru, but Marco had more screen time and was fighting pretty much evenly with him. I really don't know that Rayleigh could last as long as him though, but all I'm saying is there's more feats on Marcos side. Haven't seen Rayleigh do anything that he can't do, except use CoC which Marco makes up for w his haxx DF.
Also, no 2 characters of their calibur are mid diffing each other, it's foolish to think that the gap is even that big XD. The winner would walk away with some severe injuries, maybe a lost limb.