I would think that braking on a road bike would put far more stress on a headset than sprinting. With good form, your bike isn't leaning much in either direction while sprinting. Braking, however, puts the entire mass of rider+bike times the acceleration (thanks Newton!) of braking on the headset using the fork as a lever arm.

Though i usually disagree with Dutret, i have to agree with him this time on the point that talking about the stresses put on a track bike without infering that the bike is being ridden on a track is pretty absurd.