Go with something in between 3.5k and 5k. I had 8k come in a kit and I could barely see the road because little light sources (person behind me, someone passing me on motorway) would absorb the light and I couldn't see my own beam. Replaced with some DDM bulbs (I already had aftermarket ballasts from a company called "Alpha") that are 5k, and I can see great. Make sure if you're running them in projector housings, to lower the beam so they don't go through peoples windshields... I just fixed an issue where about 100 people flashed their lights at me, talk about feeling like a jerk.
And as a side note, depending on where you live, DDM's direct import shipping from China can take up to 2 to 3 weeks (I live in NC, it took 2 weeks.) Ships China Post Airmail, then USPS when it hits CA. But go DDM, very very cheap and I went with it cause everyone vouches for them a lot around here and the Vortex.