In both cases there is a very low probability that information would be misdelivered if the connections are properly configured. However, there is a remote possibility that information could be misdelivered as a result of equipment malfunctions or misconfiguration. With both services, you assume that you can trust your carrier's personnel.
Encryption lets only the intended recipient decode the communication.
Users may upgrade from one model to the other over the network. There is an optional management system for controlling the keys and other features of the encryption units. With any security investment, the economic paybacks are not as easily calculable as with other purchases. Like insurance, you only collect when the travesty you are trying to avoid occurs. The encryption process adds both overhead and delay.