A module of an application sends once in a while one character to another device through the SCI channel. If the delay between two characters is sufficient to sent one character at a time then the polling mode of the SCI (the AsynchroSerial component) should be used in this case.
A module of an application communicates with another device, that is it sends several characters at one time and receives characters from the device. Thus the interrupt mode of the SCI (the AsynchroSerial component) should be used in this case because when a character is received the interrupt is invoked and the underlying process of the application need not check if a character is received. When a buffer for sending is used, the characters are saved into the buffer and AsynchroSerial's service routine of the interrupt sends these characters without additional code of the application.