If the AC66U is functioning well enough as an access point, why not continue using it? You could put the FiOS modem/router into bridge/pass-through mode and then drop in a wired vpn router to distribute the load and responsibilities across each piece of gear, which I find often leads to better overall performance and stability; it also makes troubleshooting and upgrading a fair bit easier, as you can hot-swap hardware in and out as desired.
If you're proficient enough in your networking knowledge, I'd look at an ubiquiti edgerouter lite for $90. More mainstream, user-friendly, albeit less powerful, options would be the Linksys LRT or Cisco RV series at $150-75. If price is no object, a $350 ZyWall 110 is a solid buy.
