MH (Metal Halide) lamps are used exclusively for plant growth. These metal halide lamps provide a light similar to daylight while HPS lamps provide an orange light closer to the sun's rays at dusk.
The difference between an MH and an HPS bulb is also expressed in terms of heat output. With an MH lamp, it must be evacuated thanks to an air extractor in order to keep a suitable temperature for your crop.