Cain wins. While Jones retains his reach domination over Cain, he's also dealing with a high caliber wrestler who's heavier than guys he's use to fighting.
I think its closer than people give Jones credit for, but I still think Cain takes the win.
I honestly think it would be a very hard fight for Jones because of Cain's size. He probably has better technique and timing but if he takes one good shot from Cain, it's lights out because of the size disadvantage.