MH is the bulb for veg (ie growth). HPS is for flowering and fruit. Also, how old are the plants? That will play into the distance. I am not a fan for seedlings unless you can put them like 4 or 5 feet away (lost about 100 seedlings at 400w and 3.5 feet). For older plants (about 3 weeks or a month with a few sets of true leaves) you should be good at about 3 feet or so. BE CAREFUL! If they start purpling, move the light up!