depends what you're putting it on. pendants offer good heat dissipation which would benifit having it over a smaller tank, or one upstairs where perhaps heat isn't easily gotten rid of.
HQI refers to the ballast, not the bulbs. You can have single ended bulbs or double ended bulbs...really the only difference is for the SE bulbs, you don't need a glass shield, you can just blast your light straight at the water, ultimately giving you more light to the inhabitants of your tank. I've never used DE bulbs, but I think they offer less heat, and they're smaller, which gives you a slimmer looking canopy...though SE bulbs kick DE bulbs in the teeth when it comes to output
HQI ballasts will make your meter spin off the wall, whereas an electronic ballast will offer lower power consumption at the cost of output. both electronic and HQI ballasts however treat your bulb nicely, giving you long life from them. not knowing for sure, but I would give the benifit of the doubt to the electronic ballast getting the longest life from a bulb before PAR values drop and color shifts occur.
with a 250w HQI ballast and the right bulb, you can beat a lot of probe/pulse start 400w bulbs letting your virtually keep any coral you want.
depending on how much you want to pay monthly on your power bill will ultimately be your decision with what you choose, 150w or 250w (or even 400w)
forgot to mention: stay away from probe start ballasts