[rcxdude]

No, it's the acceleration. The forces are absolutely different between your two examples. The energy is the same, but in one case the energy is dissipated over a longer time, requiring less acceleration and thus force. Jerk and acceleration will tend to be correlated in collisions, but you can construct cases with very high jerk and low acceleration which are just fine and cases with very high acceleration and low jerk which are not at all safe, though the latter case will generally require a lot more energy (fighter jets and rockets, for example).

[samatman]

Airbags are the most important safety feature in cars. They don't change the rate of acceleration: the car comes to a halt at the same speed, therefore, the person inside does as well.

What they affect is jerk.

[rcxdude]

No, they absolutely do change the rate at which the person inside stops: most importantly they affect the amount of time it takes for their head to stop moving, by starting to decelerate it before it hits the dashboard (the seatbelts also contribute to this).

[samatman]

In other words they change the rate of change of the acceleration. We call that derivative jerk. Jerk is responsible for the shockwave which would otherwise rattle the brain around, it's also the chief factor in producing whiplash.