A USB 3 device plugged in to a USB 2 port will run at USB 2 speeds.
A USB 2 device plugged in to a USB 3 port will run at USB 2 speeds.
The benefit of getting a USB 3 device (where the device can take advantage of the speed) is Macs will have USB 3 ports eventually (hopefully soon) and then you'll get the improved performance of USB 3.