Its really just what you prefer. Both are proven light systems. I personally have the MH lights I run 2 250w balasts with XM 20k DE bulbs. I was worried about heating up the tank but I have NOT had ANY problems with temperature. I think that mainly applies to people who use a canopy. With my MHs I am able to run the super blue 20K bulbs to get the actinic POP effect without sacrificing the corals needs. Plus I like the shimmer effect :P