Actually, from a physical point of view, the two are identical (assuming the collision is truly head-on and not off-center or tangential). Think about it- after this inelastic collision, there will be no net movement of the point of impact in either case. On each side of the point of impact, the mass of the car will decelerate from 50 mph to 0 mph.
If the choice were between a bridge abutment and a parked car of the same make and model, the choice would be easy-- the net effect of the latter would be to take one car going 50 and one going 0, and smash them together so both are going 25 mph. (Law of conservation of momentum).
Likewise, if the other car were a semi-truck of much greater mass than your car, the abutment is the right choice. If you hit a truck head on, you not only decelerate from 50 mph to 0, but end up going close to 50 mph in the opposite direction. The effect is the same as hitting an abutment at 100 mph.
Note that energy is not conserved in this kind of collision, only momentum. This is an inelastic collision, and much of the kinetic energy is converted to noise and thermal energy.
-ccm