That question is very easy to be ansered: 4300K is the way to go if you want the most light on the road. 6000K is still acceptable but it is already remarkably bluer and everything above and beyond 6000K is just adding bling.

Now that said, please keep in mind that the after market HID kits ARE illegal, even in the US, The way you can get away with it is by installing the 4300K bulbs because that is the color temperature which ALL the OEM HID lights use. Basically you make your car look like an upgraded but OEM upgraded car. If you add 8000, 10000 or even 12000K lights you are just inviting cops to stop and ticket you. And yes they can and do do that, not very often yet, but if they look for a reason to stop you, you just gave them one.

I myself put in the 4300K in both the X-fire and the Passat and it just stock upgraded wile giving me much more light on the road.