I think metal halide gives a lot more light per watt but they have a huge drawback- they produce a lot more heat than VHO, which still produces a lot. If your tank doesn't have a chiller it can easily overheat with intense lighting. I know metal halide is best for SPS like acropora, but a lot of beautiful reefs use only VHO or power compact. I use VHO because of the metal halide heat problem (from experience).