A lot of newer cars from the likes of BMW,Audi,VW,Mercedes etc are starting to use 5000k Bulbs as people want a more blueish look.
4300k are quite white which is why i say never go over 6000k.
Depending on what kit you get depends on wether you should use 4300,5000 or 6000k
If you get a 35w Kit (from the likes of Hids4U.co.uk) then get either 4300 or 5000k
If you get the one i listed from Hids-Direct then get either 5000k or Max 6000k
Hids-Direct are cheaper and (i used to have a 35w kit from Hids4U before going to a 50w Kit) the quality is fantastic.
I have used a conversion kit now for nearly 2 years. I drive with my lights on all the time and they have never missed a beat.
Hids-Direct customer service is fantastic aswell. The first batch they sent me the Passenger side light kept flickering. They sent me a new kit which arrived the next day. They didn't even wait for me to send the old one back which most companies would.
The reason i went to 50w is for the extra light output and the claimed twice as bright as normal xenons if definately true.
I haven't got any pics of the osram nightbreakers or phillips extreme but here are some pics of the Xenons at night. Note this is from the 35w HIDS4U kit not the Hids Direct 50w kit. If i get time later i will take a couple of pics of the 50w Kit in the dark.