Metal halide lights are really white (lower quality are yellow) and tend to make things look washed out. If you ever study much about how light and water interact, the blue light tends to be all that's left as you get deeper, so there is also the thoguht it is more natural and the spectrum is better used by the photosynthetic creatures. Using some blue or purple makes it a lot more pleasant looking and enhances the colors of the fish and corals. Also, you can use the blues and purples to simulate a sunrise / sunset effect with timers.
One downside is they generate a LOT of heat.
One nice thing about metal halides (and is also true of LEDs) is that you can get a very nice shimmer effect you can't get with flourescents.
My understanding of LEDs is that to get ones that are good enough for a reef, you're going to pay quite a bit.