The MH is best for vegging as the HPS can cause some stretching. Typically people will use only MH for veg, and HPS for flower. However, I use an HPS from start to finish, and I've seen people use an MH from start to finish also. If you have two ballasts, I would just run both the MH and HPS at the same time throughout the grow. If you only have one ballast, then use the MH for veg and the HPS for flowering. Always better to be running 500w of light instead of 250w... Optimum IMO would be two MH's for veg, and two HPS's for flowering. :twocents: