Aside from being the number one and two pound for pound fighters in the world, Mayweather and Pacquiao are also both in a unique position with regards to their fight.
They have both proven themselves to be better than most of the rest of the fighters in the world, at several weight classes. With this in mind, there seems to be very little left for either man to achieve in boxing. Pacquiao already has his political aspirations lined up for after he retires, and Mayweather will no doubt start to slow significantly over the next few years.
With that in mind, an old professional wrestling line seems particularly fitting, that 'the loser should leave town' (or in this case retire) with the rest of their legacy in tact.
Should Mayweather lose
Mayweather is no doubt a big star, but a lot of his appeal is that he is unbeaten. Obviously there are fights out there for Mayweather if he loses, but the fact is that with his appeal much diminished, retirement might be a better option. I don't think Floyd will retire, after all his money problems aren't going to go away and boxing is the only thing he knows when it comes to making money. Also living an extravagant lifestyle (even throwing around fake $100's instead of real ones) makes it unlikely Mayweather will be able to walk away and stay away.
After all, the fight with Pacquiao should make him in the region of $40 million, and he isn't getting any younger for a boxer who relies on his reflexes and speed rather than his chin. At 32, Mayweather is probably still at his peak at the moment, but in another year or two he will start to slow down and get caught more often. Assuming he doesn't want to end up like Roy Jones, he needs to know when to call it a day.
Should Pacquiao lose
Whether Pacquiao wins or loses, it seems likely he will probably retire after the Mayweather fight. Not only will he have nothing left to prove anyway, but he might well be an elected congressman by that point. He had previously said that he wanted to have two more fights, presumably one of those being Mayweather and the other being either Yuri Foreman (for an 8th weight class and title) or Shane Mosley. In all honesty though, I don't see this happening if he gets elected. The biggest difficulty in walking away from the ring for many boxers is not knowing what to do with themselves. Pacquiao shouldn't feel this as much due to the fact that he will have another career to fall into. Whether he actually wins the election or not, which is far from assured, he seems set on politics in some form or another in his future.
Should Mayweather beat Pacquiao, particularly convincingly, then other than another crack at Mayweather, which he may or may not agree to and may or may not be in the contract, there isn't much left for Pacquiao either. Wins over Shane Mosley or Yuri Foreman would obviously lose some of their appeal should Mayweather unseat Pacquiao from the top of the rankings.