Well, I think it will all depend on your ultimate goal for what you want to keep.
For example,
What corals do you have now?
What kind of lighting do you have now?
What corals do you want to be able to get?
If you have several corals like zoos and such all ready then you might want to look into a fixture that also has T-5's or even PC for that matter. (To leave that a light on a little longer than when the MH is on). Also depends on what kind of room you have on top of the tank. Whether the tank is an open top, or has a canopy? Lots of possibilities.