Rayquaza is identic with its attack stat, stat that it is shine at.. I would do power up 15/15/12 first for this reason
But to think by fact.. Having 14/15/14 ray is as good as having 14/15/15 ray (its cp btw is 2076 not 2078).. And with breakpoint calc (you can find at jobspace) the difference between 14atk with 15atk is very small (about 0.3 point difference at lv40 charge move in few raid boss, and no difference at mostly boss) *)p.s: you can check breakpoint at jobspace
So, if later you have enough stardust.. I do recomended to keep power up both with 15/15/12 first and 14/15/14 later