Both ways of connecting are fast enough for the cable modem. If you use the USB connection, then you will need drivers for the modem.
If you are using linux or older windows os, then you most likely will have to use ethernet connection.
When I had my cable modem connected to USB, my USB mouse woud not work correctly when i was downloading with high data rates
(2mbit/s). Thats the only reason why I use ethernet connection.