Something to consider is that at 20, Tyson was already the number one or two contender with over 20 fights (23–0 w 21 knockouts). Ali at 20 had won all 10 of his pro fights, none against names you would recognize. To be fair, Ali had probably fought about as many rounds as a pro, as Tyson had a lot of first round knockouts.

Neither had, of course, fought world champion opposition by this point, but Tyson had wins over Mitch Green, James Tillis and Jesse Ferguson. So I think in quality of opposition to that point in their careers, while overall the average is probably pretty similar, Tyson had fought better opponents. In five more fights Tyson would be the WBA http://champion.It would be 10 more fights before Ali took the title by beating Liston.

On the other hand, ALi (then Clay) had won a gold medal in the Olympics and had over 100 amateur fights. Tyson had less than 30.

I think Ali continued to improve for longer. Cus D’Amato (Tyson’s coach and an important father figure to him) died in November 1985. In my opinion Tyson continued to improve with Kevin Rooney, but he fired Rooney in 1988, and I think his skills actually regressed after that. (Not solely due to the change in trainers. I think he by then had a very tumultuous personal life, and that contributed to his decline both directly and by contributing to the rift with Rooney). By contrast, Ali had Angelo Dundee throughout his career (excepting only two fights).

So to answer your question: I think at age 20, Tyson was closer to his peak than Ali was to his at the same age, and for that reason I give Tyson the edge. If they fought ten times, each an imaginary first fight between the two with each on his 20th birthday, I think Tyson wins 6–7, cutting off the ring and getting in close. The other 3–4, Ali controls the ring, stays off the ropes and uses his long, fast jab to stay out of trouble and accumulate points.