Do you mean Cat 6 or Cat 5e? Cat 6e isn't a thing and 6a didn't exist in 2006. 6 was certainly more future-proof at the time, although arguably 5e is still fine even today. (Super-gigabit consumer equipment didn't really exist until the last five years and it's till notably more expensive and less common than gigabit, which runs on 5e just fine.)

Cat 6 for sure. I thought there was some extra tweak beyond that, but I probably misremembered after all this time. Perhaps it was just that it was plenum-rated..