I would say to Use Shadow Sneak Here, giving you overall Good Coverage, Seeming as the main reason that several use Dark over Ghost is because of Ghost type move's in-ability to hit Normal Types, Although with an STAB Drain Punch packing, that won't be a problem.
Using Shadow Sneak Provides you coverage agaisn't other Psychics, and more particularly Other Gallade's packing similar movesets for the Bulked up Priority Kill, And Ghost types who have the Type advantage over you.
Without you being a Scarfer Shadow Sneak is the best Option.
Night Slash has a higher base power and similar Type coverage, but it doesn't cover other Gallades, except for the Neutral Hit, and it doesn't give you priority which depending on your team, you could desperately need.
While also it makes way for a good revenge killed with that studly 125 Base Attack Stat.
I think Shadow Sneak would in most situations (Including yours ) would be the best option.