1. use ethernet cuz you keep a usb spare.
2. use ethernet cuz more O/S support the nic cards than the cable modem (if you were to try another O/S that is

)
3. I've found that USB activity uses slightly more cpu power than an ethernet card would... if you've not got the quickest PC of the bunch, and you'd prefer it to stay reasonably quick, go for ethernet...
i use ethernet cable modem, its easy to use, and a longer ethernet cable is cheaper than a longer usb lol.