Like I said, anything will work for veg, but how much weed do you want? If you want the max for your efforts you need to veg with MH and Bloom with HPS. The wrong light may fool some people into thinking their plants are growing big and fast, when in reality it is making them stretch, and they will not produce the maximum yield possible. The plants are trying to compinsate for crappy lighting by opening up so the light can get into all the branches, and growing real tall to get up and over whatever is keeping the proper light from them. (In this case... light.)