1. DE vs. SE(Mogul
socket), DE pendents are more compact, for a while they were the most effective in reflecting light into a 24x24 area and had more choices in color temp and brand to select from. DE bulbs are easy to break while setting into its socket and when removing, the bulbs are typically more expensive, and the pedants are hard to ventilate heat from. SE, cheapER priced light bulbs and almost as many choices as the DE now, the larger pendents are MORE effective in reflecting light. Good SE pendents are larger and most cheap reflectors are not as effective in reflecting light. The bulbs break often if you have fish that likes to jump, as most reflectors don't have a glass shield.
2. Electronic or Magnetic(sometime called Tar and HQI) ballast. Hard one to help anyone with. First, go with Icecap if you do decide to use electronic, they are the best and Vue\Reeflux is junk. Electronic ballast is smaller, lighter and don't generate as much heat as the Magnetic ballast, it is more efficient in transferring the 115V from your wall to your light bulb. The HQI are the stronger of the magnetic ballast variety, they use more Amperage\Wattage but also drives the light bulbs harder, give out more light. They do generate significant amount heat in a reef system.
3. Wattage, this depends on what you want to keep. keep in mind there are less bulb choices for 150W SE and DE, they are not as popular as the 250's. I think the heat from both will not different much if you ventilate the pendent and ballast correctly.
4. What color cloth you like? Don't ask people what you should like, you need to see different light bulb and ballast combinations to figure that out.
5. Pendents typically are more effective in getting most light into a tank, cheap mogul "wing" style reflectors are just, cheap.. Most pendents will also fit in a properly built canopy.
6. T5 vs MH.. All depends again on what you want to keep and what is your preferred "look".. T5 lit tanks typically don't look as nice as a MH lit tank.
The best resource in MH reef lighting is all in one place, Dr. Sanjay Joshi's website.
http://www.reeflightinginfo.arvixe.com/ Spend a few hours there and you should know everything you would want to know about MH.