Not much difference in Tp-Link Archer C6 v3 vs Archer A6 v3. In previous version of C6 i.e. v2 openwrt support was present but not in version2 of Archer A6.
While currently while writing this version3 of both A6 & c6 don’t have openwrt support. Version 3 of both A6 n C6 have dual core processor instead of single core of version 2. Though most user report decreased wifi coverage in both successive models.
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. All in all it doesn’t matter, buy the one you are getting cheaper.