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
Vancefish Reviewed by Vancefish on . first big grow by twitch i have had 3 other grows on this site where i had 1 to 4 females, now i am doing a grow where i a hoping for at least 8 if not more. i started with 50 seeds of different genetics from people i have gotten over the past 4 years. i now have 26 seedlings they are under a 4 ft 4 bulb t5 lights, they are in fox farm potting soil and in the next couple of weeks they will start on a half dose of fox farm nutes i also have a 400 watt switchable ballast with an hps and i am getting a mh Rating: 5