Having had the AEBS now for over 4 years, would there be an security / hardware advantage in upgrading to a 6th Gen model instead (other than range of wireless for AC devices)?
Basically, no. The current "most secure" level of Wi-Fi security is still WPA2. Both your older Extreme and the very latest, 6th generation, model support this option.
For both wired and wireless security, your VM Superhub may offer additional router firewall features not found in any of the Apple routers. I would compare them before making a decision.
FWIW. In my case, I needed some additional features and opted to use a Cisco as my "main" Internet router. I then use multiple AirPort base stations to complete my home network.