[Delphinus]

As Brad says there is no simple answer to your question. Here are some of my thoughts, though...

HQI = Halogen Quartz ... um ... I forget what the "I" stands for. Index maybe? HQI is used to refer to the double-ended halide bulbs (similar to a halogen bulb). Basically, it's just the arc tube and two contact points. Unlike mogul base bulbs which look like a really big incandescent bulb. Since double-ended bubs don't have the glass globe, they don't have the built-in UV/heat sheild. They need to be sheilded by a glass cover otherwise there is risk for UV burn and damage to your eyesight. These bulbs require a different ballast. I.e., a 250W DE requires a HQI ballast to fire properly. You cannot take a standard 250W ballast and use it to fire a double-ender. There is no such thing as 400W double-ended as far as I know. If you hear of a 400W HQI ballast, it's just a trade name.

EYE - I think this is a ballast to run Iwasaki bulbs. Iwasakis are mercury vapor bulbs, but can be fired off a standard halide ballast.

Electric They're all electric. Oh! you mean electronic. :P Expensive, but supposedly draw far less electricity for the same amount of light intensity (or more according to some claims) out of the bulbs. They tend to be more expensive, but, they are claimed to be easier on the bulbs so the bulbs may last longer. The ballasts are said to not produce the annoying electrical hum that the traditional capacitor-and-coil ('cap-n-coil') type ballasts produce. I have my cap-n-coil ballasts in a different room than my tanks, on account of the heat and the noise they produce.

Unfortunately to start comparing all the different options available all against each other is a bit of a loaded question. There are just so many different variables. A lot of differing opinions out there, all valid. It just depends on where you're coming from and what you're used to!

FWIW, I've seen spectacular colours of corals under the DE lights (Jamie's tank and Carpentersreef tank, are two examples of tanks that run using DE bulbs and the colours of corals in their tanks are amazing.) But, it is expensive. For example, Big Al's is selling an Aquaspacelight (2x150W HQI plus a few blue PC actinics) for the trifle retail price of $3000. You can go with PFO pendents, get the same light for a lot less coin than that, but, they are still more expensive than their mogul counterparts. There was a rage with HQI's about a year ago when MFA2002 came out because Sanjay's article comparing halide bulbs showed a 150W HQI putting out more lumens than a 400W. This was a cooked comparison though, because the 150W had a reflector and the 400W did not. It just goes to show that a good reflector can make a bulb seem more bright without even changing anything else.

I'm keen to try electronic ballasts some day, but I haven't yet gone that way on account of the purchase cost. But, one day I think I'll give it a go. If Kyoto gets in as it looks like it will, then we're not going to be liking our hydro bills very much, and we're going to be forced to look at any alternatives that keep that bill from skyrocketing if we want to stay active in the hobby..