MH metal halide is the white/blue perfect spectrum for VEGATATING a plant.
HPS high pressure sodium is orange/red spectrum. for FLOWERING.
both can be used for the entire grow.
if you are going to use just one go with HPS
i grew side by side same strain two plants. 1 with HPS and 1 with MH the HPS grow was on crack the plant was loving it while the MH was a little slower and less yield. now i have to plants surrounded by on top a 200w HPS on the sides 1 hood of double 150w flourscents and the other side is a 200w MH let them live the life and increase the yield . if i had the cash i would went for a 400w or a 600w but oh well.