While Xenon bulbs imply the HID type (a gas tube with an electrode at each end that requires a ballast and igniter), there are Xenon filled halogen bulbs all over the market. The latter slightly raise the Kelvin color closer to the HID's typical 4100K for OEM auto applications, but they are just halogens. Marketing hype on a lot of dot.com sites makes it difficult to discern sometimes.
Xenon HID lights will have an electronic box called a ballast, and will have either an external igniter (the D1S and D1R bulbs) attached to the bulb, or will be built into the ballast as most are. For aircraft applications, the various vendors out there typically use the D1S bulb found in some Audi's, Porche - they have the igniter in the bulb base, and supposedly radiate far less RF on light ignition due to the lack of sending the 20,000 plus volt initial charge down a wire from ballast to bulb like on D2R/S types. I never could find an application where the more common D2R/S bulbs were tried to see if noise was an issue.
HID bulbs are rated in Kelvin in terms of their color - around 2800-3000 is warm (halogen), while 4100K is what most OEM autos are, but there is a big market in the Fast and Furious crowd for the more blue higher Kelvin bulbs, up to 8000K. Stick with 4100K if you are rolling your own. Another thought is to provide contrast against a daytime or dusk bluish sky, the warmer the light, the more contrast. Here, a halogen of similar luminosity may have some superiority, but I could not find objective data. I found anecdotes where pilots saw the warmer halogen easier than the HID when there is ambient light out, but that problem can be solved by using a wigwag (and yes, HID's can run on a wig wag - there are various vendors that sell them). At night, there is no issue - a 35w HID will rock.
In terms of reflectors, you can try and adapt a common PAR36 reflector (PAR=parabolic aluminized reflector, the 36 refers to the diameter in 1/8 inch increments - 36/8ths=4.5 inches in diameter). This reflector is used in the Duckworks kits that are used in the leading edge. Builder Pete Howell in Texas converted a set of these reflectors (which you can buy alone from Duckwork's), and got the bulbs/ballasts from an auto via ebay. Current prices on the latter for 2 lights are over $250, and figure $80 in parts from Duckworks (reflectors, mounting plate, lens).
Creativair and Van's/Duckworks sell a small HID light that is 2 inches in dia based on the MR16 size used in many commercial and residential applications - it is designed to fit in the wingtips. Those have the igniter built into the ballast I believe, but Creativair's owner wrote that he wasn't aware of noise issues.
Overall, I agree with the beauty of HID lights. The light is close to the sun in Kelvin, has superior road visibility (I've been driving behind them for 3 years in my Acura), uses only 35 watts in most apps to achieve more brightness than even 100watts in a halogen, and has a long lifespan, perhaps 3000 hours. Disadvantage is the cost, and a little more weight due to the ballasts (maybe 1.5-2 ilbs for a pair).
Overall, I have not concluded which way to go, but am leaning on converting some PAR36 reflectors with an auto salvage HID bulbs/ballast, but frankly the cost is not much better than the Creativair MR-16 style ones. The latter have a relatively narrow spot pattern (12 degress if I recall), and I want a taxi light with 35 degrees or so, while the landing can be more of a spot (8-15 degrees). I may buy some offroad halogens (PIAA, Hella, KCLites etc) that have published spot patterns and convert to HID. Still researching the whole thing. Then there are LED position lights...another topic!
