It all depends on what you are trying to get out of your reef. I'm assuming you are doing stony corals. If not, no reason to do MH. Assuming so, do you want growth with less coloration, or slower growing with brighter colors? 6500K bulbs are the fastest growth, but by far the ugliest colors. Almost noone uses these for reefs. The next major step up is the 10000K bulbs, which will still have excellent growth and more color than 6500K, but still blands the corals a bit. I personally use 14000-15000K bulbs. Still better growth than 20000K, but better color than 10000K. I like to think of them as the best of both worlds. If you don't care about quick growth and only want it for aesthetics, 20000K is the way to go. Very blue and very deep coloration of the corals.
I recommend VHO actinic supplement also. I have 2X110W VHO super actinic and 2X150W DE(double ended) 14000K/15000K and it is well over what is needed for my 55G. Almost 10W/Gal. I personally like this choice, just watch out for heat. Mine is called Acrolight by PFO. Very nice system, but somewhat expensive.
