4300k is the truest white and has the highest light output. The higher the K the lower the light output and the more color gets thrown in. Though some might dig the colors, they can make night driving a bit strenuous to the eyes and might attract Johny Law (HID retro fitted into a none HID designed light housing IS ILLEGAL).
I went with the 35 Watt system 4300k. It has a huge amount more light output than the stock Halogen bulbs. I didn't see a need to go 55 Watt, as they will just degrade the chrome finish in your housings faster due to more UV and heat.
~Mike....