They are both just as good all though many will tell you that Ethernet is best. The main reason for this is that a USB connection will take more processing power from your CPU that an Ethernet Card. The Ethernet connection is no faster than a USB connection (Well it wasn't for me and believe me I well and truly tested it)
I've run both USB and Ethernet on a 3 PC home network using ICS and I could see no difference.
Go with which ever works best in your circumstances.