HQI ballast will overdrive lamps giving off more PAR (photosynthetically available radiation) generally. But HQI tend to use more electricity, and run hotter. I'd imagine lamps have to be replaced more often as well. Here is a link w/ some PAR values for different bulb/ballast combos, including regular electronic ballast vs an HQI ballast. http://www.reefkeeping.com/issues/2007-04/jb/index.php
Double ended lamps will need a glass shield to absorb harmful UV
rays. SE, or mogul based lamps, have the uv
filter built in to the outer envelope.