USB is a peripheral interface, and it connects your device to a computer.
Ethernet is a network interface, and it connects your device to a network, theoretically allowing any device on the network to talk to it. (For better or worse.)
I would vote for ethernet. Or some other network interface like the wireless (Bluetooth, WiFi) suggestions. The cost of wireless interfaces keeps going down, and is (paradoxically?) somewhat independent of "technology" at the DIY level. (That is, a BT module, a Zigbee module, and a WiFi module are all pretty similar in price.)