Which Mewtwo would you max out?

Both are fine. I maxed a 2268 because I got it first, but if I had gotten a 2258 first I would have maxed that. You won't see any difference in performance between them the vast majority of the time. If you want something that maxes out at a higher CP, pick 2268; if you're obsessed with 15A, max 2258. It really doesn't matter though.

Personally I don't find Mewtwo terribly useful most of the time so depending on your RC/dust situation and what you prioritize raid-wise, you might want to hold off on powering up either one of them. I have one maxed that mostly just collects dust and wouldn't consider powering up a second unless something comes out that warrants it (or by some stroke of luck I get a 100%, because 15-15-15 is cool). There should be some good legends coming down the pike for Gen IV that may be better uses for your RC, or Kyogre/Groudon with their signature moves.

Neither.... wait till we see what the ultra bonus is no sense in maxing out a Pokémon that might be outclassed in two months.

The 98IV duh. Nothing a 15A Mewtwo can do that a 14A can't. It's attack stat is already bonkers.

98>93. Easy peasy

98, because 14 does not differ much from 15.

There won't be a substantial difference between them.

However, I'd wait to max either out right now. If you need to make room in your inventory, by all means drop Rare Candies into Mewtwo, but wait on powering them up until after the Ultra Reward unlock. The best that could happen is that you could get a technically more useful Mewtwo if the rumour of a Mewtwo day (like Moltres day) end up being true, and the worst is that you wait 2 weeks to power up an awesome Mewtwo.

Normally I'd argue the 15a, but after checking a multitude of breakpoints against raid bosses Mewtwo is normally thrown at there aren't any notable ones that a 14a would miss. Unless you desperately need that extra point of damage from its charge move the 2268 would be the winner in my head.

Like others have said, it may be best to wait and see what this ultra unlock is. Without factoring in the Psystrike situation, if it returns as a raid boss there's a (slim) chance of finding the elusive 100%. Psystrike shouldn't be a huge factor against Mewtwo anyway, it has better utility right now using Shadow Ball or Focus Blast than it does Psychic.

let the professor take a look.

314 points vs 315 points
Almost zilch, the def and hp will help you more.