MH- has been tested, and works. They can grow coral exceptionally well and color them up just as well. - Downside? Heat, Electricity.
LED- Relatively new to the Reef aquarium hobby. They can grow coral exceptionally well and color them up (IME with a Radion, the color is not as good and lacks the intense fluorescence that a 400watt Radium can achieve). Downside? Very low resale value considering the speed at which LED technology is advancing, as well as our understanding for the required light spectrums that coral require.
My 2 cents.
Regards, Walid