I think that factory ones are usually around 4500K, which are a bit whiter and brighter than 6000K
6000K refers to the colour temperature, not to the brightness. So for example, 8000K are more blue than 6000K, and 10000K will be sort of purple. Because of the difference in colour, those ones with a "higher" colour temperature are actually *less* bright.
Go for 4500K if you want super-bright HIDs. They will be a little bit brighter than 6000K. A lot of people go for 6000K though because they're still bright, and have a very slight blue tinge which looks kinda cool :D