5000k IMO. They are the brightest and the most legal looking. 6000k will look more like the typical European car headlight that you're thinking of.
One thing to double check/understand is that you may not have high beams depending on your vehicle's lighting setup (1 bulb or 2). You don't need them with the HIDs (or if you have a 2 bulb setup, your low beams will be much brighter than your high beams

), but just be sure that your state (or anywhere you are moving) doesn't check highbeams during smog checks or anything like that.
If you get them, try not to directly handle the bulbs. Finger oil isn't great for them; wearing latex gloves might not be a bad idea when installing them.