A recent study tried to answer this very question. Researchers thought that people in faster moving cars would have more severe whiplash. In fact, whiplash--as measured by the amount of neck movement after injury--didn't change with the speed of the collision. (The speeds of the colliding cars were only about 40 kilometers per hour apart, and the accidents were described as "minor.")
Age and body mass played a bigger part in whiplash. The older and larger the person in the car, the more likely he or she was to have less neck movement after the accident.