Mosley wanted the fight badly. Cotto stated its the defining fight of his career. Come to think about it, which fight brings the more excitement?
Despite the impression Cotto's style can give off, he's a generally conservative fighter tactically for the most part. Particularly when he's facing a capable and dangerous offensive opponent. Mosley is much much more of a freewheeler and fighter willing to take chances. For me it's Pacquiao/Mosley zero question. But if someone considers Pacquiao/Cotto more exciting because Cotto is more vulnerable to getting rocked or KTFO, I can appreciate that too.
Pac - Cotto. Mosley murders pac if he is still fresh (also coming off the margo win will have his confidence through the roof).
I say both because all three fighters are offensive minded much more so than being defensive minded. Just for the sake of choosing one, I'll pick Pac/Cotto.