Im with Mick, 4300k is much better light, wonce LED technology starts to advance to making different light coulours it will be even better. also there are a big difference in HID ballasts, I recently put my 70W quality kit into my IPFs over the cheaper 55W kit it had in there and WOW what a difference, light out put has nearly doubled.
The main reason for using 6000K or 8000K is the price. Just like HID bulbs 6000K is the cheapest to make. If you ever look on ebay you will see HID kits a lot of the go from 4300K to 6000K and miss 5000K which is dearest to make.Dont cree already make a range of leds in different kelvin ratings? Its just a matter of lightbar manufactures to make the move from 6000-6500k down to a more usable 4300-4500k. I dont think this is gonna come anytime soon unfortunately due to how well they work now, people seem to perceive the blue tint from current hids and leds to be a desired trait.