If you're in an old car with no crumple zones, my intuition says it's better to hit a modern car because then you also benefit from the other car's crumple zones. Colliding with another rigid car would basically be like hitting a brick wall. I think the effect on the driver ends up the same in both cases.

If it's two old cars with rigid bodies colliding, it's exactly like hitting a brick wall. Even if the car itself is unharmed, the driver isn't. It's how quickly you stop that makes the impact dangerous, and in a car like that you stop almost instantly.

On the other hand, when two modern cars collide, there's 2x the crumple zones, so the impact is the lowest there.