Depends on what you want. The 4100k bulbs are more white and brighter than the 6000k. The 6000k have a slight purple/blue tint to them and are not as bright as the 4100k. So if you want the brightest go with 4100k and if you want the cool looking purple/blue light go with 6000k. As you increase kelvins the color of the light changes more towards a blue/purple color, but you lose max light output compared to a 4100k bulb.