I'd have to say Murray is more capable, but I think Janks is more likely.
Murray has to get through at least 3 dedicated professionals in Nadal, Federer and Djokovic, in addition to a host of up and comers.
Janks' competition are the Williams sisters, who seem to have dedicated themselves to tennis, but still don't play enough, Shrieks, who is injured, Ivanovic, Kuznetsova and perhaps Dementieva. All of who suffer from nerves, and none of whom can hold their serve in a big moment.