Khan has a bigger built in fan base than Bradley. In terms of popularity, and marketability, it isn't always about wins and losses. Tim Bradley is simply not a box office draw. Losing to Pacquiao isn't going to change that, especially if he doesn't perform well.
Frankly, much as I like him, if he beats Pacquiao-even decisively, getting the official win and everything, he still won't have staying marketability as a big star, I don't think.