My take on this comes from my favorite grow guide:
"You can run
- hps light through both stages of growth (ok).
- mh light through both stages of growth (ok).
- run a mh light through the veg phase of growth followed by hps light through flowering (very good).
- run both mh and hps light through both stages of growth (best).
When grown with hps light only, the marijuana plants I grew were taller and thinner than when grown under mh only. That meant a smaller harvest. Plants grown with mh light only, tended to take longer to flower but the harvest was larger.
However, many growers I've asked said they have had better results using hps, if only given the choice of using one type of light. This is something you can experiment with yourself, if you are interested.
You can't use a standard high pressure sodium bulb in a metal halide fixture, but you can use a metal halide bulb in a high pressure sodium fixture of the same wattage."