Answer
The collision in which the two cars rebound is expected to be more damaging.
Work Step by Step
If someone is unfortunate enough to be in a car crash where the cars rebound, the change in momentum of the car (and its occupants) is greater than if the cars had stuck together. Each car was not only brought to rest, but the force was great enough to reverse its momentum and send it backwards.
This greater impulse resulted from a greater force or the same forces over a greater interaction time, so it’s likely that the “rebound crash” was more damaging.
