Until the tank is 18-24" high or more you can do the same with either. Metal halides are best for taller tanks where you really need to punch the light down through a lot of water. Even then you can use T5HO and simply keep mushrooms, zoanthids, etc. on the bottom. Watt per watt you actually get about the same amount of light, the metal halide just concentrates into a very small area of output and the T5HO spreads it out along the whole length of the bulb.