Besides there chemical makeup (metal halide uses a metalic gas while HPS uses a salt) both can oeprate at variable voltages with th use of a transformer in the fixture, both operate under high pressurized gases. The major difference is the color of the light output. HPS turns a orangish yellow while metal halide produces a brighter whiter light. HPS found its niche with streetlamps while metal halide is more often used indoors in factorys, stores and shops. Your application is going to dictate your lighting.

