ORIGINAL: xenon
Let's face it, the standard halogen offering in the 944, even with the supplementary driving lights, is pretty weak. 1980s technology. The last four cars I have had have all had factory-fit xenon headlights (bi-xenon in the last three) and they are incomparable with halogen, especially out in the sticks where I live.
The first priority on the 944 was therefore proper HID lights, and not those chavvy blue-tinted bulbs either!
The problem was that up until recently it was difficult to retrofit an H4 application. Most cars these days have separate bulbs and lamp assemblies for low / high beam and tend to use H1 or H7 bulbs rather than the 'old fashioned' dual filament H4 bulb. You could get H4 based bulbs but you would loose the high beam. Anyway, now you can get an H4-based bulb with dual discharge tubes, so you can have high beam too, i.e. true bi-xenon.
I'm very pleased with them - a huge improvement over stock.
Regarding colour temparature, I really wanted 4300K as this is what a standard D2S bulb (as fitted to BMW, Audi, Porsche etc) runs at, and provides the best OEM look and the best performance. The higher the colour temparature, the less lumens produced. Over 8000K you may as well stick to halogen unless you simply want the blue/purple look. Anyway, 4300K were out of stock so being an impatient sod I took the 6000K - as I say though, the light output is so much better.