Murray. A lot has changed since the last time they played, IMO. Murray is playing the best tennis of his career and is at a different place mentally. He knows he can beat anyone, especially on hard courts.
Nadal's biggest advantage, IMO, has always been mental. I think he has lost that against Murray outside of clay. If the mental advantage isn't there, I think Murray is the better player on hard courts. Doesn't mean Nadal cannot beat him, but I don't see him as a favorite like he would have been in the past.