I have always used the interrupt method. There really isn't that much more code involved compared to polling, and while the ISR is waiting to run you can get some other work done.

Your ISR can be written to let the main program know when a message to be sent has completed transmission. Knowing when a received message is complete is more complicated, unless the message includes message length or the protocol used specifies message length or time limits...otherwise just let main know a received char is available.