As was mentioned, there are currently no USB3 audio interfaces... and none that have even been announced.
The best USB2 units yield excellent performance and low round-trip latency.
You'll want to connect most USB2 audio interfaces to a USB2 port.
There's no advantage connecting to a USB3 port.
Some USB2 units (FastTrack Ultra 8R) simply won't work when connected to a USB3 controller.
Unless you're using a Z series chipset motherboard, the USB3 controller isn't integrated into the chipset.
The USB3 ports are provided via 3rd-party add-on controllers.
Liken the scenario to using a non-TI chipset Firewire controller.
Increases the odds of compatibility issues... or reduced performance
ie: You can connect a RME Babyface to a 3rd-party USB3 port... and it'll work. But you can run heavier loads (glitch-free) at the smallest ASIO buffer size when connected to a USB2 port (integrated into the chipset).