When fighters hang around as long as Jon and Stipe it gets very hard to forecast. Does Stipe walk in and have nothing left, a complete old man? Does Jon, with his gazillion fights, training hours, and injuries walk in diminished?
The only thing I'm confident about is that neither is what they used to be. Jon Jones probably wins, he certainly has more ways to win and he is a certified killer and legend, but I don't know. I think ultimately whichever guy walks in feeling physically better wins and, again, that is probably Jon.