You can do the entire grow with one bulb.

Yeap... that's right... from veg to flower with MH.
Or, from veg to flower with HPS.

It is *better* to use MH for veg, then change to HPS for flowering.
It is *even better* to use both MH and HPS for flowering.
It is *even even better* to use both MH and HPS for both veg and flowering.
It is *even even even better* to use the full spectrum of the sun.

If you have to pick one bulb for your grow, I believe it's a 'general consensus' (w/e) that an HPS bulb would be the one to have.

Peace