Join Date: Apr 2005
Location: Los Angeles, CA, USA
Can't comment on the 35W vs. 50. Some eBay listings claim 'better compatibility' with CAN bus systems (the rationale being 50W is closer to the factory 55W system), and 'more light output' when using the 50W kit vs 35W. Personaly, I installed a 35W aftermarket HID kit on my K1200LT before, and couldn't have been happier.
As far as the color temperature (measured in Kelvin): 4300K is pretty much the boundry between pure white and blueish haze. Also, I believe 4300K is the DOT approved color temperature (which you may or may not care for). Higher K degrees, like 6000-8000 will produce blueish color, while lower K temperatures will yield yellowish light.
Also, while blueish tint may seem 'cool', the actual light output and the perceived brightness decreases as the K temperature increases.
IMHO, the sensible range is 4300-4800K, but I'm sure others will chime in with meaningful opposing views.