I am alternating lights in the Veg room at the moment. I have 2 MH 1000 on the outsides with a 1000w HPS in the middle. I also use a400w MH and HPS 400w in the Veg room. I try to rotate the plants around to get a good dosage of both, some get better exposure and I try to give the plants a day or two in each blended spot. Like was said earlier HPS give stretch and MH less node distance, but also my plants under mostly MH are more willowy and fleshier, and easier to train, whereas the HPS plants are sturdier, but harder to train. I'm trying to get a good mix of both in my plants. It is mostly to your personal preference. Some ppl swear by only HPS, some use MH in veg only and flower with HPS, and others use MH the whole way. Find what you like best and what works best for you, giving you the best results.