Hello mashup .................. Ahhh an easy question, my favourite type

Buy yourself a ballast that can take MH an HPS bulbs.
Why ? ............ Simple because the MH bulb is the best for growing vegitive growth, it produces mainly light from the blue end of the spectrum which is what plants need to promote vegitive growth.
Now the advantage of having an interchangable ballast is that when the time comes to induce flowering you can simply put in the HPS bulb which produces red light which is a plants favourite for promoting flowering.

mand