Jones. He has the wrestling to deal with Cain, and even threaten taking Cain down himself. As well as a big reach advantage...
Cain is a very fast HW, but not sure he is faster than Jones.
It would be a tough fight for both of them. Not an easy win for Jones.
Jones vs. JDS or Jones vs. AO... once he moves up and puts on some solid mass would be great fights.