New tp link Archer c6 vs old Archer c7 v4

Which device will be best for openwrt ? Should I go for a new TP link Archer C6 V3 or a one year old TP link Archer c7 v4. I won't use these devices wireless interface I will use them only as a VPN client (sending all traffic though socks 5 ip tunnel). So for this uses which one I should buy ? ( The price gap between Archer C6 and archer c7 is around $10. C6 will cost me $10 more than 1 year used Archer c7 v4)

For what you are doing, the C6 V3 is hands down the better device. Dual-core MIPS 880MHz vs single-core MIPS 775MHz. The C6 will handle VPN traffic much better.

A Netgear R6700v2 with the same mt7621 chipset was my first introduction to both OpenWrt and Mediatek. I was so impressed with Mediatek that I standardized on it.

Agreed. With the C6v3 you are looking at up to 130 MBit/s via Wireguard tunnel whereas the C7v4 will max out at 60-70 MBit/s. Since you don't plan on using wifi, where the C7 outperforms the C6 in regards to range, go for a C6v3. Just make sure that you get a v3 and not v4. The latter is not supported and won't be in the future.