I've read that DE bulbs are more intense than mogule based bulbs. DE bulbs does not block the UV
light generated by it so you will need to have a glass shield. Mogule base bulbs will filter out the UV
but you should still have a splash guard.
The 250W DE has more penetration depth than the 175W SE bulbs. If you got a deep(as in verticle height) tank i would suggest getting the 250W.
I had a 10g tank with 150W DE. The distance from bulb to sand was approximately 16" apart. I had a blue mushroom rock directly under the MH
and the population sky rocketed over server months. Mushrooms are supposed to be low light corals but these seem to love intense lighting. I wouldn't worry too much about frying your corals because SPS
, Clams, anemones will love it the extra light. And if the corals react badly you can always relocate them to a less intense area.