I was just about to post about that topic, Firestorm =P
Anyway, Grass Knot is almost always a better option when a non grass type is using it, since you'll mostly be hitting those who are weak against grass, and those tend to be heavy guys.
On grass types it's closer and you might want to go with the more reliable Energy Ball since you'll be getting STAB and might want to hit some lightweights who only take neutral damage and don't want to use Leaf Storm.