I expect Mayweather to win with some ease.
Early on, Pac will deliver flurries of punches that will get the crowd going nuts, but when slowed down you'll see that 90% of them never connected. By the mid-rounds, Floyd will have Manny chasing him and boxing at his pace.
Manny has had 4 legendary fights against Marquez, with 2 wins(one should have been a loss), a draw, and got KTFO by Marquez in the last one. A couple years ago, Mayweather outlcassed Marquez in every possible way in the ring.
Prediction: unanimous decision for Mayweather. 117-11, and 2 judges scoring it 116-112.