Assuming equal mass and an inelastic collision, the missing information here is, "How well is the wall fixed to the ground?" If the wall is immovable and perfe

Assuming equal mass and an inelastic collision, the missing information here is, "How well is the wall fixed to the ground?" If the wall is immovable and perfectly fixed to the ground, then the car/wall collision will indeed have a greater effect than the car/car collision. But if the wall's connection to the ground is negligibly weak (e.g., an actual brick wall), then after the collision, both the car wreckage and destroyed wall will continue forwards at 50 mph, until some other force slows them down. In this case, the effect (change in kinetic energy) is the same as the car/car collision, not counting the impact of crumple zones. In the former case, the kinetic energy of the car relative to the ground is relevant only because the ground is exerting a massive force to keep the wall in place.