MH is Metal Halide (the light that gives off the blue color)
HPS is High Pressure Sodium (the orangish colored lights, mainly street lights)

mrpibb you can use them both throughout your entire grow but thats gonna be twice the electricity bill.
You dont wanna use a hps for vegative growth because the orangish spectrum of the light doesnt promote leaf and branch growth, but it goes great with the flowering and producing buds on the plant.
Cannabis grows the best with those 2 different spectrums of lighting in your grow
i'd say your best bet is to use the metal halide throughout the vegative phase of your grow and the hps during the flowering phase.
Its the best way to maxamize your yields with those lights.
and yes it is really worth it