Hi Stuart,
Personally, I think one (1) Radion XR15w Pro would work really well over your 60G cube, especially if you plan a "typical" mixed reef with SPS on the upper-half of the tank.
With that being said, a single G3/G3 Pro would be more than sufficient -or- you could always add a second XR15w Pro if/when you filled the tank and needed some extra PAR.