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. Forget about moving one without taking it apart. Yeah the difference is the crashpad and the frame. Fortunately, we know in advance exactly where on the frame this is going to happen, so it can be heavily reinforced and padded.