Mayweather - Marquez will be closer than what most people say.
Mayweather has a much better defense, better speed, great counter attacks and he's naturally the bigger man. People talk about Marquez beating Casamayor. Casamayor doesn't present the same skillset as Mayweather and doesn't defend as good as him.
Marquez, on the other hand showed that he had a lot of heart, motivation, confidence and also great timing. The blueprint for Marquez will be to try what Castillo did but he's not strong enough for that. Unfortunately for him Mayweather doesn't have trouble with smaller fighters. What can beat Floyd is a stronger big powerful man who will attack the body constantly. That's why Cotto and Margarito were tough opponents Floyd didn't fight.
Marquez has something like a 35% chances in this fight.