Since they have the same Base Power and Hydreigon's base Special Attack is higher than it's Attack, Dark Pulse is generally more powerful. However, if you have a low Sp. Attack stat due to EVs, Nature, etc. Crunch may be more powerful, but Dark Pulse is more powerful if those stats are not changed, due to Hydreigon's Base Stats on his Attack and Special Attack:
Attack Special Attack
For the EV spread running a Special set, I suggest 252 Sp. Attack, 252 Spd, 4 HP. A classic spread on Hydreigon that works well.