There are no 802.11ac radios that are as well-supported as the ath9k 802.11n radios. All 802.11ac radios require closed-source firmware. Qualcomm-Atheros, MediaTek, and Marvell 802.11ac platforms are getting the most open-source attention. QCA chips are really popular and the drivers are pretty mature, MediaTek's firmware is flexible enough to make it one of the more hacker-friendly platforms, and Marvell's radios are often paired with their really powerful (by router standards) processors.

Unless you really need the throughput of 802.11ac or a more powerful processor to keep up with faster WAN speeds, ath9k 802.11n devices are still a pretty good choice.