USB or Serial modem?

Serial.

With a serial modem, it plugs in to the serial port of the computer and requires no software. Serial port modems are meant to be more reliable and our testing supports this.

Serial modems connect to COM1 or COM2 and can transmit around 6 SMS messages per minute.

With a USB modem, you will need to install a driver to create a virtual COM port in Windows. These COM ports will be COM4, COM5, COM6 or higher.