Depends on the brand and the bulbs you use...
There ARE wide spectrum MH bulbs-- only so-so from what I have heard...
There are also single-spectrum bulbs, where you use 1 for veg, a different one for early flower and a 3rd one for the last 3-4 weeks of flower... The single spectrum's are (IMO) a waste of money.... we tried them in Veg alone, and they reduced our plants vigor and yields IMMENSELY!
We went back to CFL's for young plants and HPS for the older ones-- are switching to T5 for older ones once we have the $$$....
But HPS is FOR FLOWERING but can be used in veg if using a wide-spectrum bulb.... MH makes buds fluffy in flower.... FROM MY EXPERIENCE WITH THEM ONLY.....
Use it for veg-- or better yet- get either a dual bulb fixture that holds a HPS AND MH at the same time-- or a switchable ballast that you use for a MH in Veg-- change out the bulb to an HPS and flip the switch for 12/12... worth the extra $60!!!
But- YES- you then have to have a fan and ducting to cool the air flowing thru the hood of the light- or the room heats up and the light has to be further above the plants than optimal.... so figure that into your cost!