Eras. They're best not compared really. Tyson or Ali? Shankly's Liverpool versus Fergie's Manchester United?
It's impossible. Nobody can definitely say who was better, who would have won.
What about lining Carl Lewis up against Usain Bolt? That's a different matter. Definitely, we can say the Jamaican is quicker. He has covered 100m in 9.58 seconds and 200m in 19.19s. Lewis posted 9.83s and 19.75s.