From what I've read(in the lighting section of this forum).

MH put out a higher blue spectrum and are best for Vegitative growth. There is also some belief that the UV put out by these produces more resins.

HPS put out a higher RED spectrum thus simulating the Fall sun. Which in turn is needed to flower.

It has been said running both the entire time is best. However next best is MH to grow and HPS to flower