In my grow room I am running two 1000watt lights one HPS other MH. Ive been veging for 2 weeks and notice no difference in lighting. The plants that are directly under HPS and the plants directly under MH are all the same size and same amount of bushyness. My understanding is that the MH bulb came before HPS, what I mean is I belive that marijuana has been growing under MH and flowering under MH before HPS was invented. I never noticed mother nature changing her light just changing the amount of time of light.:stoned: