i used to use mine through usb, but found it had a tendancy to drop the connection when left on for a long time (over 24 hours or so)
since using it on a nic instead ive not had problems like that, so id recommend the nic route
note that i hate usb anyway though
theoretically neither is faster, since your cable modem is at best transfering 1mbit/sec of data, well usb1 can do upto 12mbit/sec while network connections over ethernet are either 10mbit or 100mbit depending on the card used
either way both are faster than the cable modem can possibly use at present
the only thing that might make a difference is if you had a lot of usb devices, espically those like scanners, hard drives, digital cameras and printers, as well as the modem on usb, which you were using all at the same time, since thats obviously going to cut down on your available bandwidth for the modem.
thats more likly to be an issue when usb 2.0 has become the standard, which is alot faster anyway (480mbit/sec i think is a figure ive seen quoted)