Answer and Explanation.
The serial console is a connection that allows a person access to a computer or network device console and it is accessed over an SSH connection. That is secure shell (SSH) which is a cryptographic network protocol for operating network services securely over unsecured network.
While USB console connection is use of port in connection where the USB cable determines which COM port to connect to.
The advantage of using the serial console connection is that it is easier to connect to the router or switch using a serial console connection unlike with the USB console connection.