I see Cain as favorite
I don't see on Overeem the same TDD ability of Cigano, and after the Overeem vs Werdum fight I don't think that Reem's striking is that good. I think that Cain has the capacity of take Overeem down and work the GnP. However, on his last fight, Cain made many times the mistake of putting is head down, and if he does that against Overeem he will probably get a knee on his face.
If I had to bet, I would say Cain by TKO (ground and pound)