They are both good at using energy if properly designed, so that comparison doesn't hold up. Trebs will deal with unused energy by re-lifing the counterweight. Catapults slam into a stop. I don't know when the Romans used the Onager and Ballista, but I think it was well before the Medieval period. Go to Russel's websight for more accurate dates, I could easily be off by a few hundred years.
Catapults hurl lightweight projectiles very far and very fast.
They pack a lot of power for thier size and weight. They are much more mobile than a Trebuchet and throw farther. Trebuchets hurl extrememly heavy stones only just far enough to be useful in a siege. But the total power available was almost unlimited if you had enough time to construct one outside a fortress. They can smash through walls that lighter stones would simply bounce off.