I am wondering which one to use.
OK, so there are no stat differences, but their hidden abilities are different.
Male Meowstic has Prankster and Female Meowstic has Competitive.
Second is their movepool. The male has more support-ish moves which works well with Prankster while female is more orientated on attacks.
I think that is all, other than their appearances, which you can see in the source.