Old USB maximum speed is 12 mbps.
USB 2.0 maximum speed is 480 mbps.
Standard Ethernet runs at 100 mbps.
Gigabit Ethernet runs at 1000 mbps.
Work it out for yourself.
...
USB is 12 Mega Bits per Second. Ethernet goes up to 100 Mega Bits per Second. Cable Modem only gives 1 Mega Bit per Second. So, both is OK.
...
"Use Ethernet unless your cable modem supports USB 2.0 as well as your computer."