Why Ethernet instead of USB?

[Neale]

Yes - Ethernet gives ground separation between the two ends (good in industrial situation where bits of kit may be on different mains supplies) but also helps reduce noise issues, and the cabling is twisted-pair and differential signalling, also good for noise rejection. Fundamentally more reliable, which is why an Ethernet connection can be much longer than USB. USB can be pretty good but Ethernet is undoubtedly better. However, in most home workshops, USB is probably fine, but if the cost difference is not very much, I would always pay a bit extra for Ethernet.

Depending on how your pc/network is configured and how happy you are setting up network addresses and so on, an Ethernet motion controller might need an extra network hub, or its own network interface in the pc. I run my CSMIO Ethernet motion controller on its own separate network, and have a wifi adaptor in the pc to talk to the house network for file-sharing, etc, but that's just to suit my own network situation (and I'm happy playing with network parameters anyway)