We found out that the onager is powered by torsion and the trebuchet is powered by gravity. The trebuchet is a lot bigger than onager and it fires a lot further than the trebuchet. The onager is faster and more powerful than the trebuchet. The trebuchet has a higher arch than the onager because it has a longer arm than the onager.
As there are differences we found out that they are both are designed to do some heavy damage. They both are made to smash through walls with stuff like boulders and occasionally objects that can be set on fire but the onager is made to take down walls while the trebuchet is made to go over walls. We used tennis balls and they both went for around 30 meters. That was the strongest thing we had access to. Both also have a sling, a place for the projectile to sit, a ball most of the time but sometimes something similar to shotput.
In conclusion we found out that there were a lot of similarities and lots of differences. I wonder if the onager and trebuchet would be designed differently now days?