This is what I said in the Floyd-Mosley thread, just substitute Floyd's name with Pernell's, because I feel the exact same way about Whitaker-Mosley:

Floyd by decision, 116-112 sounds about right.

Mosley, for all his obvious physical gifts, never developed a well-rounded style. He was always a straight ahead offensive fighter. He never used his speed and athleticism to develop a great defense. He also has a tendency to become one dimensional with straight right hands.
Mayweather would win because he is a little bit faster than Mosley, and more importantly, has the technical precision and defensive genius that Mosley lacks.
Mosley can only fight going forward, Floyd can be effective going forward or backward. He just brings more versatility to the table.
Floyd can hit Mosley and avoid getting hit that much in return, Mosley can't hit Floyd without leaving himself wide open to counters.