I had 6000k in my golf (came in the lights I had) worst damn things ever at night... swapped them for 4300k the next day and was amazing. Then I didn't like the slight 'yellow' so I went to 5000k and they are brilliant at night and work a treat.
I opted for 5000k when I recently did my Bi-Xenon retrofit (check my build thread) here's the lights and output... it is very clear, and brilliant at night. but as said i wouldn't go any more than 5000k (pure white).
(I'll get a better night shot if you'd like?)