The whole "common opponent" thing really means nothing. Pac and Mayweather are different fighters. Manny is going to be more aggressive and KO more people. It is what it is.
You can't go by the old "A>B, C>A, so C>B" theory. Compare the common opponents of Ali and George Foreman in the same way, and you'd conclude that Ali wouldn't have made it past 2 rounds with George.

Styles make fights, and the thing that was the most telling for me was how Manny got outfoxed by Morales the first time and JMM. The things that these guy's used to give Manny fits, Floyd has and does better than both of them combined. Manny has a puncher's chance for sure, but conventional wisdom (IMO) always suggested that Floyd would be victorious.

But this is all academic at this point imo, because Floyd vs Manny will go down as one of boxing's biggest flubs. They blew the opportunity already. Back in 2009/early 2010, Manny looking like an unstoppable punching machine. Floyd looked as sharp and quick as ever.

Now, Manny has been beaten and looked lackluster in recent efforts. Whether or not you think Bradley won, you have to admit that Pac didn't look like a world beater by any stretch. And he got another gift against poor JMM. You even got Pac talking about retirement in the near future.

As far as Floyd goes, I think you can see that he is on the decline. His legs aren't what they used to be, and he's getting hit a lot more in recent fights. He's getting up there in age.

They had the opportunity to do this when both guys were peaking, where the hype was SO BIG that the winner would have an argument for the greatest of all time. It's still an interesting fight but it's lost a ton of luster.