As Neale said...

But what he did not mention is that the actual communication protocol standard for Ethernet is also much better than that for USB. Ethernet protocol is faster, more reliable and robust because it has a built in error correction algorithm which is not existing in USB. This alone motivates the little extra cost quite well, even in a home environment. Personally I had the UC300USB before, but even if it is also a very good controller, it had reliability issues, whenever I started the vac it crashed the communication because of the spikes the vac motor caused. This was pretty frustrating, so I bought the UC300ETH and that solved all the issues. Of course, the ETH is a bit more complicated to set up if you are not familiar with LAN configurations, but it is definitely well worth the effort. It is also offering huge flexibility, since you don't have to have a wire connected between the CNC and the PC, you can run it through Wi-Fi LAN, as well as through a LAN switch, or if you prefer, you can run it via a direct LAN cable, just like if it was USB, with the exception that the UC300ETH must have it's own power supply, as opposed to the USB controllers, which can take the 5V from the PC via the USB.