Our existing network (about half the total space) uses Cat 6 wiring installed 3-6 years ago, before Cat 6A was widely available. We have Gigabit Ethernet to the desktop. GbE is good enough most of the time, but some heavy data users and some network backups could use more bandwidth.
Which means I should put in Cat 6A in the new space to "future proof" the network. Which costs more. Which might not be in the budget. And we definitely don't have the money to rip and replace the existing Cat 6 cabling, creating bandwidth "haves" and "have nots" in the building.
Cat 6A cabling is much thicker than Cat 6, which will likely cause problems with conduits, ladder racks, cabinet punchouts, bend radii, etc.
By the way, POTS (Plain Old Telephone Service) runs just fine over 2 pairs of Cat 6 cabling terminated in RJ-45 jacks. RJ-11 cables plug right in with no problems. Any jack in any office can carry either POTS voice or Gigabit Ethernet data, with a couple minutes in the telecom closet swapping jumper cables.
We probably won't decide between Cat 6 and Cat 6A until we get cost estimates in a few months.