For which raids is Mewtwo a hole in your lineup, and does Mewtwo reach a breakpoint with 15 attack in any of those raids?
If not, the 14/14/14 seems great to power up. Ultimately I'd imagine you will eventually power up at least a couple more Mewtwo, so you'll be waiting a very long time to get several better ones anyways.
I'd say bring it as high as you can while saving enough candy to max one out if you roll better (including rare candy). It already has the best moveset for helping take down future Mewtwo anyways!