HPS is warm lighting, red spectrum. Aids in bud formation & trichrome production. Causes stretching, elongated stem growth. MH is cool lighting from the blue spectrum. Encourages healthy leaf growth, short stong stems & plant respiration. HPS in vegetative growth can cause long enternodes, weaker stretched stems that sometimes can't hold the bud weight, & a much taller plant that may out reach your grow areas height. You end up with much more stem & a spindly plant. If you have a choice, DON'T use any WARM light for vegetative growth.