The Wi-Fi where I work is spotty at times and spotty in certain locations (like where my cube is :-( ). I won't get into why it's still like this after months of complaining but I want to use the USB Ethernet at my desk since it's more reliable and then fail over to Wi-Fi if I unplug the USB to go to meetings or something.
I found this link on the apple stackexchange site but it's not doing what I think it should be doing, which is connect in the order specified in System Preferences. It appears that it's connecting to both and using both sources (Wi-Fi more than USB).
In this screenshot, we can see via iStat Menus that both interfaces are receiving traffic, although, most traffic has been going thru Wi-Fi even though I've been plugged into USB for about the past 45 min to 1 hr. Hopefully there's a way to connect strictly via USB when I'm at my desk and Wi-Fi when I move around.