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the difference in color: 6500K will be white with hints of blue while the 4300K will be in the white with hints of yellow (much like stock HID's)
the difference in Usable light output: the 6500K will produce much LESS light output than the 4300K (which is the best if you really want to see the road very well)
also, if you decide to get the 6500K...see if they have an option for a 55W ballast instead of the 35W most HID Kits come with. That will increase your light output with out sacrificing much of the "looks" you may be trying to go for.
I would recomemnd 35W kits. 55Ws are more powerful but they do have realiabiltiy issues and the bulbs would not last as long; this is not including the possibly legality issues with it as well for being too bright.