Do SCs come into the equation? Basically you'd get double the SCs for the CX trip.
But if they don't, think of the fun you could have with £750. Plus there reportedly seems always to be a reasonable chance of W->J op-ups, and as a WP/OWE you might well attract the eye of the upgr*de fairy, so you've got some chance of ending up in NCW for some or all of the trip at the lower price.