I'll vote for Iron Mike because of one of the biggest element modern boxing have perfected : Body toughness.

Even if Lee was a prodigy in his time, he never fought a 250 lb beast like Tyson was. Sure a martial artist usually got the advantage again a boxer in 1 VS 1, but the difference is too big. Tyson's punch almost have the same reach of Lee's kick, and we can all agree the fight would end as soon Tyson connect something. Since boxer are about the toughest athlete nowadays, I don't see how Lee could bring him down unless kick his balls or something. Of course, he could try to wear down Tyson with a "hit-and-run" tactic, but I'm pretty sure Lee would be the one to break first.

That is also without taking into account the "real" fighting experience of Lee.