Hps has more yellow light then MH and more lumens per watt. A MH has more par watts which are usable watts of sorts. The MH has a more balanced spectrum of over 3200K while the HPS is about 2700K. Sun light is 5150 if I remember correctly. Plants will be more compact at the end of flowering by using a MH as yellow light streches plants. People use HPS as it speeds the flowering process. Yellow light breaks down cloroplasts which are converted into sugar then starch which is what the plant eats. Blue light builds them. The more food the better for the plant interms of growth speed not health. They acually have a bulb which is a MH that is 5500K. I have yet to try it. This is a pic of the 400 watt version found at http://www.esuweb.com/products/243Co...500_KELVIN.htm