For competitive: Leaf storm is better. Frenzy plant has to recharge and any move that has a recharge is unviable. Plus with its hidden ability, contrary, Leaf storm boosts serperior's special attack instead of lowering it.
For In-game: Stick with energy ball, Giga drain, or leaf blade. Niether of those moves are good for in-game because 2-turn attacks like frenzy plant always suck and lowering your special attack(which is what leaf storm does) isn't helpful.