Ingame I would go with Petal Dance. The game allows you to switch after you get a KO, meaning that even if you are mid Petal Dance you can switch out. This allows you to ignore the main weakness of Petal Dance, which is being trapped in a move that is easily resisted. Leaf Storm also has this advantage, doing major damage and KOing the foe and then switching out, but it has less PP, making it less useful for ingame.
For competitive use, Petal Dance is worthless because you are trapped in an easily resisted attack. Leaf Storm is a good choice for sets that require more of a hit and run style, such as the Choice sets or any set making use of Healing Wish. If you're going with a Quiver Dance set though, ignore both Leaf Storm and Petal Dance and go for Giga Drain.