I think Whedon and Singer are roughly equals in the directorial department. But in different ways. Singer doesn't wear humor on his sleeve the way Whedon does, but can do some really fun and funny stuff, like Quicksilver. And Whedon is more sentimental and heartfelt, which you really got watching Firefly/Serenity IMO, but also things like Coulson in Avengers. That has its charms, but isn't for everyone. I think Singer is more straightforward, and probably better at serious drama, going back to his earliest stuff like Usual Suspect, Apt Pupil, and even Superman Returns. In the action department I think it's about even. Neither is a master of film, but both are quite good at what they do IMO, and both seem pretty well suited for the teams they are currently focused on.