I plan on maining either Mewtwo or Shadow Mewtwo in Pokkén Tournament. It depends on which one I find more fun to play as, and, seeing as Mewtwo is Standard while Shadow Mewtwo is Technical, I'm definitely leaning toward Shadow Mewtwo. I normally dislike playing as characters that are average in all areas, so, to me, Mewtwo might feel a little boring to play as. I mean, if Mewtwo was the only form of Mewtwo in Pokkén Tournament, I'd probably be willing to overlook his Standard classification, but, since Shadow Mewtwo is also an available choice, I might wind up using the latter.
That said, Shadow Mewtwo also appears to use several moves taken from other characters, such as Reflect from Gardevoir, Flamethrower from Charizard, Earthquake from Garchomp, and Thunder from Pikachu, while Mewtwo has his very own moveset consisting of more "Mewtwo-ish" moves, like Hyper Beam, Psycho Cut, Focus Blast, and the Elemental Punches. Granted, Shadow Mewtwo also has Psywave, which appears to be a highly versatile move unique to him, but Mewtwo's moveset consists of entirely unique moves. If Shadow Mewtwo feels more like a conglomerate of other characters than a form of Mewtwo himself, then I'll definitely wind up using Mewtwo. The latter's ability to link Fire Punch, Ice Punch, and Thunder Punch together, in particular, really stands out as interesting, in my opinion. Of course, Shadow Mewtwo's Psywave has, like, 5 different ways to attack, so that's quite interesting, too...
The point is, I'm super-indecisive right now, so it's probably best to wait until I get my hands on the game before making my decision
. Aside from Mewtwo and Shadow Mewtwo, Chandelure looks fun, as well, especially since his Burst Attack is basically murdering you. I highly doubt I'll main him, though.