Given the potential wireless bandwidth of 802.11ac. What are the proposed wired connection options for the APs. Multigigabit etherchannels? 10Gb/s sfp+? >1Gb/s over Cat6?
In the first wave of 802.11ac, a single 1 Gbps link is sufficient. Wave 1 is 1.3 Gbps, but that includes the substantial 802.11 protocol overhead and is a bidirectional number because 802.11 is half-duplex. For any new wiring for 802.11ac, I'd put in two cat 6 cables for maximum flexibility going forward, though.
Cat6 versus 6a isn't what's important, it's getting two cables into the cable plant. The second wave of products will potentially reach 3.5 Gbps, so you'll want sufficient backhaul capacity to accommodate that. I wouldn't stress about the exact specification; just make sure you have two cables that can support Gig Ethernet plus power.