I realize that Whedon has writing credits for what is infamously one of the worst lines in movie history (I think you all know which one I'm talking about).
But I'm going to defend him for a bit: That line would have fit Perfectly in an all-Whedon script. I could have seen something like that in Serenity, which is a severly underrated film.
Whedon is an amazing talent, and a huge X-Men fan, albeit one that seems to relate more to the female characters, which may throw an X-Men film off balance. That aspect would be good for the Emma Frost Spinoff.
And while his dialogue is very quirky, smart-allicky, and witty, I don't know if that would fit well with X-Men. There are always lots of great Zingers in his scripts, even when someone dies. He's good at making us laugh in a darkly-humorous kind of way, which would be great for an X-kids spinoff, but not X4.
So I'd hire Whedon for X-Kids or Emma Frost, but not X4.
But then again, Burton has his quirks, and Batman somehow worked.