As Fabled Asian mentioned, depends on what attacker you need for your team.
I prefer Venomoth though as it has access to Quiver Dance + Sleep Powder so you can set up with ease and a great ability in Tinted Lens that doubles the damage of moves that are not very effective.
With Ariados, you do have some great moves in Swords Dance, Shadow Sneak, Sucker Punch, Cross Poison/Poison Jab, Sticky Web and its Egg Moves to back it up, it's low speed with not so great HP and Defenses makes it really vulnerable.