ive seen people use a mh all the way through and a hps you can do it the difference is that your plants need more of the blue spectrum during veg (mh) and more of the red during flower (hps). what you could do to try and get better duel spec is add some cfls the white ones not the ones that put off the orange light as this to is the red spectrum for flowering. but if you are going to only have one light hps is the way to go. also is your ballast digital most of those can run both mh/hps. gl and happy growing
a lot of hps bulbs are dual spectrum, they still tend towards the red end of the spectrum but have enuff blue to do the veg, mh is better but i just had a full hps gro and its looking good. if uv got to pick only one bulb its really got to be hps.