From what I understand, Xenon is the gas added to Halogen bulbs so that they can burn brighter (hotter and whiter). Xenon is expensive and therefore companies like to 'boast' about it saying rightly how much better Xenon is over Kypton.
I remember, but couldn't find that thread, but I didn't looks so hard...
As a moderator, I'm not going to close this thread.
Let's see how popular it is and whether we can get a better answer then mine to your question!
Al