Tyson doesn't get enough credit for his early defensive skills, as taught to him by Cus D'Amato. His peekaboo style was perfect and his leg strength allowed him to actually duck below punches (see the Quick Tillis fight) with ease. I think Tyson would have proved a bigger threat to Ali than Frazier did - greater strength, speed and skill. Ali won two of their three matches, so I think Tyson could have had equal success. I'm talking Tyson circa 1986, not 1996 or, God forbid, 2006.
For longevity at the top and ability to win against adversity though, Ali is miles ahead of Tyson.