I personally dont really like either fighter but I believe that this is a dream match up in favor of mayweather for a few reasons. He's bigger in terms of reach and weight. Reality is that manny will probably weigh about 145 for the fight while mayweather will be closer to 153 on the actual day of the fight.

My second reasoning came from breaking down there styles and opponents. Now i dont believe that ABC logic works in sports. i mean muhammad ali lost to joe frazier and ken norton while george foreman knocked both of them out. but the closest thing to a mayweather that pacman has fought is jmm. manny's bread and butter is that one two and he tends to lunge a bit with the left and once marquez got the timing down he effectively controlled it and counter him numerous times and was able to hurt him while they were at the same weight. so im gonna imagine that a better counter puncher, whose bigger, and faster should have at least equal success or greater.

Thirdly, manny is a southpaw. Common boxing knowledge dictates that your most effective punch is the straight right against them. This again benefits mayweather because not only the reach factor but because he throws less jabs that your average boxer and is more straights than the average fighter.
Also much of manny success has come against opponents where his speed advantage was clear and away particularly against hispanic fighters ( not tryin to say there slower than any other fighters) who generally fight with no head movement and move straight at there opponents. this has allowed manny to move in and out with his superior speed and bounce around and attack from different angles with his one two. He wont have those advantages against mayweather and if the much slower marquez could slow him down and clearly frustrate manny in both fights i have to believe that mayweather who is as fast, much greater defensive ability and even better countering skills is just gonna pick him off with straights taking advantage of his reach and not letting manny line up to blitz with his left. Mayweather wont do much damage but he'll frustrate manny and dominate on the punchbox % and the score cards