Here are some similarities we thought of. They both launch because of the forces that they generate, for example the trebuchet uses the gravity of the lead weights and the onager uses torsion from the twisted rope (this happens because the ropes want to untwist).
They may have their similarities but they also have differences. For example, the onager has more power so goes further and it also launches quicker but the trebuchet launches slow because doesn't have much velocity.
We noticed the onager launches faster we think this is because it has got more velocity but goes lower. We think it goes lower because the arm is smaller also it goes lower if we pull it to the first hole.
The onager uses holes to launch the projectile further, the furthest will shoot the furthest because it gets more force.