I would only recommend Shadow Ball Mewtwo as a last-ditch effort if you’re lacking a strong lineup of Ghost/Dark-types (Gengar, Origin Giratina, Chandelure, Darkrai, Tyranitar, Weavile, Hydreigon, etc). Ghost coverage has more overall utility in the raid scene than Psychic coverage does, so if you find that your lineup is lacking, you may consider SB Mewtwo. Note that many of the ones listed above outclass SB Mewtwo against Mons weak to Ghost/Dark, so if you have those Mons or can get your hands on them via trades (minus Darkrai), invest in them first.
Otherwise, I agree with @hkn. Psystrike Mewtwo is the best Psychic-type currently out there. It’s only outclassed by Shadow Mewtwo that’s been Elite TM’d to have Psystrike. If you’re into master league PvP or Rocket Grunt battles, Psystrike Mewtwo is also more valuable than Shadow Ball Mewtwo.