Then there isnt much to add.
"HQI" is actually a misused term in the hobby. In the hobby it is generalized to refer to all Double ended lamps.
Double ended or "HQI" lamps are a type of pulse start lamp that are spec'd for the Magnetic "HQI" ballast.
Mogul lamps come in three types, Probe start, Pulse start and "HQI" rated pulse start.
If using magnetic ballast, they different mogul based lamp types are all spec'd for a different type of magnetic ballast ... probe start ballasts for probe start lamps, pulse start ballast for pulse start lamps and HQI for HQI rated mogul based lamps. The double ended lamps are all spec'd for the same magnetic ballast, a magnetic HQI ballast.
Electronic ballasts will drive virtually all lamps, mogul or Double ended /HQI although not to exact specs ... in general they overdrive probe start and some pulse start lamps and underdrive
"HQI" rated lamps. But they are very popular and work fine ... they allow you to use any metal halide lamp type on a single ballast as long as the wattage of the ballast and lamp matches ... 250 watt ballast with 250 watt lamps etc.
Both mogul or double ended lamps work fine. The reflectors have come a good ways so reflectors are made for both types that allow them to be very effective and provide uniform lighting throughout the tank. One lamp base type isnt "better" than the other. There are high par and lower par lamps in both types. Bluer and whiter lamps in both types. Good quality and lower quality lamps in both types. Some reefers end up choosing one type or the other simply because the lamp they like is only available in one base type. For example, the 250 watt Radium 20K is only available as a mogul based lamp (HQI rated).
Choose a good pendant/reflector !!! and whatever lamp type you choose, you will have quality halide lighting for your reef.
Good article with the basics on lamps and ballasts ...
http://reefkeeping.com/issues/2007-03/sj/index.php
