MH is great to have in addition to HPS in growing mj, as it adds the much needed blue/white spectrum to the lighting... It also helps keep internodal spacing shorter, making for bushier plants, and is purported to give better outcomes in females to males when germing... It ALSO penetrates deeper into the canopy and helps make for a more balanced spectrum, and in my experiences, happier plants, with more resin production, because of the UV-B that they emit, causing the plants to produce more trichs, which is their natural response to such light...... I wouldnt grow under HPS or MH alone, as Im a believer in a mixed spec... It just does better things for one's grow...