From what ive always been told mh (cool spectrum lighting) provides tighter internodal spacing, the tighter your nodes, the higher the yield when it comes time to flower. The reddish hue provided by an hps imitates what the sun does when mj plants would naturally flower outdoors.
I personally use an hps just because its all i have, and have had no issues with yield or any funky problems people may claim happen with an hps in veg. I do use 3 cool cfl's just even out the light spectrum a tad tho.