Not 100% but I'm pretty sure its the spectrum of light given off. Can't get real scientific about it but basically metal halide(MH) puts out a better spectrum than PC. Since MH does this, you can keep more critters under it.
The difference is in how the lamp works.
Fluorescent are sealed tubes filled with gas and a little mercury. When the electricity is turned on electrons will migrate through the gas from one end of the tube to the other and some of the mercury turns from liquid to gas. As the electrons move through the tube they collide with the gaseous mercury atoms and emit light.
A metal halide uses a arc tube inside the outer bulb. The arc tube is filled with a gas mixture of gas, mercury and MH salts. As the lamp starts the gas mixture vaporizes and like fluorescents emits light. This so called single point lighting emits a much stronger light than fluorescents.
Both types use a phosphor powder coating on the inside of the bulb to determine color. These phosphors get worn out through time and the spectrum will shift... this is why we have to change out our lights so often.
It's the same wattage(ie 250W of PC or MH uses the same amount of watts), but MH supplies a better spectrum for your corals, etc to grow. It's sorta like driving a economy car or a sports car. Both will get ya where you need to go, but the sports car can get you there quicker