Which is better to use for a cable modem, a USB connection or an Ethernet card?

USB port has a maximum data rate of 12 megabits per second
(Mbps). However, of that available bandwidth, an individual
USB device can use only up to 6 Mbps. And if you have several
devices attached to the USB ports and they pump lots of data,
this competition may further lower the data rate.

An Ethernet card, on the other hand, connects directly to the
computer's bus. Ethernet cards come in two flavors: one that
accepts 10 Mbps, and another that accepts 100 Mbps.

If you are on a cable modem late at night with no one else
sharing the line (see How Cable Modems Work for a discussion
on sharing), the cable modem can run at data rates faster
than 10 Mbps. In normal circumstances, however, a few
megabits per second is far more likely. Taking this into
account, you could install a 100-Mbps Ethernet card if you
want the best possible peak performance.

Also, an Ethernet card is likely to be more consistent, since
it is dedicated to network traffic (as opposed to a USB
connection, which may handle traffic from a number of
devices). One difference is the ease of installation. To
install an Ethernet card, you have to open the computer
casing, while the USB option takes 5 seconds to plug it in.