Not much difference in Tp-Link Archer C6 ver3 vs Archer A6 ver3 which is currently offered in market. Only one minor difference is Archer C6 firmware is made for mostly european markets while Tp_link A6’s firmware is specifically made for US market mostly.
Currentl version-3 of both Tp_link C6 & Archer A6 both have openwrt support. Version 3 of both Tp-Link A6 n Archer C6 have dual core processor instead of single core of version 2. Though most user report decreased wifi coverage in both successive models.
All in all it doesn’t matter, buy the one you are getting cheaper. I would recommend get whichever one is cheaper at your time of purchase. You can check price of both versions with below links.
In previous version of C6 i.e. version 2 openwrt was supported while version 2 of Archer A6 didn’t supported it. Comparing to older version is not justified as older versions are hard to find specially online where most vendor are sending v3 models even on v2 listings.