Well,
UV-B RADIATION EFFECTS ON PHOTOSYNTHESIS, GROWTH AND CANNABINOID PRODUCTION. MH and CFL emit a little UVB Light, and HPS does not. If your MH lamp is in an enclosed air or water cooled fixture, the glass or water will absorb the UVB light, so you need use an unenclosed MH or 6000k - 5200k CFL for veg.
The small amount of UVB light that you get using CFL or an unenclosed MH lamp isn't worh the loss of potential flower growth. Your better off getting 2400k CFL for flower and they will emitt some UVB and you will be hitting the flowers with the right spectrum of light.
The best thing to do would be to buy a UVB emitting light, like a reptile light to use in conjunction with HPS or 2400k CFL