I think Floyd would win by decision. He’s virtually unbeatable.
But I would be rooting for Manny.
I tend to agree. Manny had a really hard time dealing with the counter punching of Marquez (in all three of their fights) and Mayweather is probably a better counter-puncher and definitely much bigger than Marquez. Manny's punch just isn't that devastating at the higher weights. I think the fight would be a lot like the Mayweather-Cotto fight: Manny would have his moments but Mayweather wins.
If Canelo Alvarez isn't rushed into a Mayweather fight and doesn't outgrow the division, he might be the guy who could finally topple Floyd. Give him a couple of years and the 23-24 yo Alvarez might make Mayweather look his 38-39 year-old age (like he did to Shane Mosley). The rumored plans of James Kirkland, then Miguel Cotto, then Manny, then Mayweather make sense for Alvarez in my opinion.