Im wondering

Okay, i've been wondering, about the effects or benefits, because I read that in a "In a perfect world, you'd have a HPS and a MH"
Now If I have a 400W HPS. What would be the benefits of also getting a 400W Mh light.
If anyone could give me some info, or insight.

a metal halide light has more blue spectrum in it , its better for veg... where as the hps has more red spectrem and is alot better at flowering then the metalhalide. if ure not vegging for a long time i would just go with a hps