From raw performance standpoint - I think it's a wash (I think FIOS provides a Gig wifi router) and you could get most of benefits (see below) if you use ac66u as an access point :
- almost unbeatable WiFi Range of AC66
- ease of administration - including guest wifi, usb/printer sharing
- usb storage/printer sharing
Now if you use AC66U as a router - you get to play with OpenVPN (very secure and fast protocol for remote access to your network), easy to setup remote access to usb storage and a free DDNS service.
Speaking of VPN - ask me for details if you have T-mobile service. In my case I had to make some changes to APN to make vpn work
btw: If you have AC66 - I highly recommend to download and install latest firmware - Asus is quite proactive with them.