The only real benefit is improved WiFi performance which will improved transfer rates between your computers and other devices in your home. 802.11ac in the Extreme can do up to 1300Mbps, the previous generation WiFi (802.11n) can generally do up to 400Mbps in ideal conditions. Additionally, AP Extreme takes advantage of a thing called beam forming. Beam forming adjusts the signal direction to maximize throughput to a given active device rather than indiscriminately broadcasting the radio signals. This can also help overcome 5Ghz distance issues to a degree.
Since your ISP service is likely the slowest point between you and the outside world, even the 802.11n will likely deliver the same speeds as AP Extreme for surfing and downloading from the web.
If you do a lot of internal file transfers or stream video content, say from Mac\PC to Apple TV, the ac WiFi might give you better results. Make sure the WiFi radios are turned off on the ATT router to avoid interfering with the AP Extreme signals.
The other potential benefit of Extreme is a USB drive can be connected to it and used for central Time Machine and\or file sharing, or Printer Sharing. If there is a USB port on the ATT router, it may not support Time Machine, but could support file sharing and\or Printer Sharing.
