Considering that Chesnaught has really high attack, Brick Break is the better option in-game, as you will be fighting a lot of trainers consistently. Brick Break has higher PP, better accuracy and no draw backs. I don't know of any trainers that use the screens, but Brick Break might come in handy.
If it's the female Meowstic, Psyshock; imo you'll be using it against Pokemon that are weak to Psychic types anyway, like Toxicroak. Note that Psyshock might also come in handy against Florges since it has really high sp. def. I think that the male Meowstic might be better off with Psychic though, since it's not as offensive based so it can just set up a screen or two and deal STAB damage with a slightly stronger move.
