Who is the better writer?
Oshii is more notable as a director than a writer but he's not bad, certainly better than Urobuchi.
Urobuchi isn't as bad as /a/ likes to make out, at least his stuff is entertaining.
A better question is how would an anime turn out with Oshii's directing and Urobuchi writing the script?
So many hipsters in this thread. Just cause butcher's popular doesn't mean you need to hate him.
His stories actually have substance and he writes fight scenes that are more thought out and strategic than 95% of all the mindless action anime out there. He's also not afraid to kill off characters, and despite popular opinion he doesn't just liberally do it. Those three things alone put him above most of the industry.