Hi
David - purchased the previous AP Extreme model a few months ago w/ a USB 2.0 port, but was curious about the speeds of these different USB vs. Wi-Fi protocols, so looked at a few Wiki articles - quoted below are the maximum USB 2.0 vs. 3.0 speeds and the Wi-Fi n vs. newer ac standards (on the Apple store site, the rates stated are somewhat different).
If a printer were attached to the USB port and prints 'wirelessly', the USB type would make no difference since the printer speed would be the limiting factor. Now if an external HD were connected and an 802.11n network was being used, then again probably no difference.
Time Capsule I believe is also a 'wireless' backup option for multiple computers (correct me if wrong?), so again w/ USB 2.0 vs. Wi-Fi n, likely similar to an external HD. So, I guess the only advantage of having the USB 3.0 port would be if combined w/ 802.11ac - I would suspect an update in the future to USB 3.0 as Wi-Fi ac becomes more of a standard. But just my 2 cents - Dave
USB 2.0 - 480 Mbps
USB 3.0 - 5 Gbps
Wi-Fi 802.11n - 54 - 600 Mbps
Wi-Fi 802.11ac - 0.5 - 1 Gbps