Eh, I'm not thrilled by Brett Ratner...after all he did the Rush Hour movies. But he also did the adaptation of Red Dragon that was actually decent. Jury's still out, but I'm reasonably certain that the third movie will at least be entertaining.
I would feel more comfortable with Bryan Singer because in directing both X-Men movies, he cemented the fact that quality can be injected into super-hero franchise films before the newest Batman flick, before Sam Raimi lovingly adapted Spider-Man. As for Singer's overall credentials, his "The Usual Suspects" still ranks as one of my all-time favorite movies, and I'm looking forward to his take on Superman.
As for the importance of the script, I'd have to say that a good movie can't be made without a good script...BUT, a ham-fisted director can make a terrible movie out of a good script. This is the caveat in anticipating any film.