I would go for 5000k. I've had 6 and 5, and 5 is more natural white light rather than 6000k which looks synthetic. 5000k is also brighter than 6000k, and not as harsh too. Remember, your headlights aren't designed for HID's and will scatter light around, dazzling oncoming traffic. The more natural the light, the less harsh it will feel to oncoming drivers. I also set my beam angle (using the rotary knob inside) to position 2, which helps cut down the glare from it (the higher the numbers, the more it points to the floor)
As for which bulbs to change, I only changed the normal dipped beam. I see no point changing the high beam. Firstly, HID's take a good 10 seconds to warm up to full brightness; you'll notice when you turn your lights on you get a flash, and then the light slowly warms up. This is pointless on main beam as you're forever turning them on and off. Secondly, once you've got a decent set of HID's in the normal dipped beam, you'll find you don't often turn the main beams anyway.
One of my 5000's went a couple weeks ago. I'd noticed that it was changing colour, getting to the point of going almost orange, and then one night it just kept flickering. I had a spare pair of bulbs (that were listed as 5000k, but are more like 6000k) so I chucked them in, and noticed how much brighter they were, because the ones I'd had in for a year or two prior had slowly dimmed. Over this winter, with the older bulbs in, I was using the main beam when driving through the twisty country lanes. Switching to new bulbs, I just don't need the main beam, and when I do turn them on, there's actually only a little benefit to them - no extra light, but it throws a bit further (because they aren't shielded).
I'll be buying a new pair of 5000's soon, as the ones I have in currently are more like 6000's, and in complete darkness, there's a level of noticeable blue, but when you get into street lit areas, that blue turns green with the orange street lights and normal headlights on the road. Number plates and road signs take on a greenish hue to them.