I personally think that it could go either way. If Dempsey drops Tyson first then it's all over. Tyson doesn't have the mental make up to come back from a knock down. He would start to freak out and either go cannibal on Dempsey (:D) or just quit. If Tyson drops Dempsey first then I think Dempsey would have only a small chance of coming back, as Tyson has proven that he is just as good a finisher as anyone in boxing.
If I were to put it at odds, I'd say that if the fight lasts under 6 rounds, then Tyson has a 60 percent chance of winning, but if it goes over 6 rounds, then Tyson only has about a 20 to 30 percent chance of winning. Overall, I favor Dempsey to win it, but I'd give him about a 60 percent chance of winning.