I just need to know which is better for Sword/Shield Competitive.
Go with that one, because:
Dynamax: Due to this, many Pokemon suddenly become much bulkier than they generally are, so you'll need some serious break through power which Nasty Plot gives. In case your own Mewtwo ever gets Dynamaxed, that'll give you the Max Guard you might need in a pinch versus opposing Dynamaxed Pokemon, something you lack access to as an all out attacker.
With STAB Psystrike/ Psychic, you generally hit neutral matchups really hard, specially at +2. The difference in the damage output is negligible if at all, and in fact at +2 you'll probably outdo super effective damage at +0
With plenty of defensive Pokemon around, you won't have too much troubling setting up Nasty Plot. However it'd be great if you could get your other Pokemon to support it by means of spreading Sleep or Baton Passing a Substitute (if you have access to that) and so on.