Be sure to choose the same watt bulb as your ballast is rated for!  You cannot run a 400w bulb on a 250w ballast nor a 150w bulb on a 250w ballast etc.  For best coral growth & aesthetics (& what I use in my facility!) I recommend using 10K bulbs when dedicated actinic bulbs are also present.  MH only systems should use 14K bulbs.  20K bulbs, IMO, makes a tank look too blue (a deepwater affect & most of our corals come from shallow waters!).  Though I don't recommend 20K bulbs I offer them because this is your hobby.