Actually a study was done (by Jeff Mohr) that showed the hitch transfers more of the energy to the vehicle because it is mounted to the frame where as the bumper has a degree of crumple to absorb some of the impact.
I agree with his study, the hitch transfers more of the energy to the frame increasing the whiplash. BUT, I was rear ended in 2008 by a semi on the interstate and think the hitch is what let me be here today. The added strength of the hitch added to the rigidness of the frame stopped the rear end and back hatch of the SUV I was driving at the back of the front seats. Everyone who saw the vehicle said I should not have survived, I'll never know if the hitch made a difference or not.