On some of the HID websites they show a graph with the light output (in lumens) vs bulb rating.
So if you want an OEM look (pure white) with the best possible light output go for 5k.
Going for 6k (which is what I went for) results in slightly less light output than 5k but with a 'modified' look. The light output is white but with a blue tinge around the outside. It's not noticeable on the road generally but you'll see it on motorway road signs or shop windows.
6k HIDS4U Photos I really don't recommend 8k, they will have a blue light onto the road and significant drop in light output compared to 5/6k.
When I bought my car it had 10k's in which I really didn't like, I found them quite tiring on the eyes.
I have a feeling that light output is more important to you than the lights looking 'cool' so i'd go for 5k
Hope that's of some use