Its a completely useless argument. The difference in enviroments that the two fighters competed in are too much to overcome and find a happy medium to judge this question on. The cage/ring difference alone makes this argument impossible. Both fighters are(were) very dominant in their own enviroment, but in Fedor's case, his success didnt translate well at all into the cage. Having watched Cain's fights, I feel his style would not translate well into a ring at all. For the most part, I think wrestlers will always be better cage then ring fighters, which speaks volumes to the points brought up about the pride fighters success against wrestlers listed in several posts above. This is also the reason I feel a lot of the pride fighters didnt translate as well to north american MMA as was expected.
All in all, what Im trying to say is home field advantage wins this hypothetical fight, and more due to adaptation then skillset.