The CM19A is a RF sender and receiver only. It uses the included TM751 to send power line signals. It can not receive any power line signals. Has no real time clock and memory to hold downloaded macros. The computer and AHP has to be running all the time for it to work.
The CM15A is a interface that can send and receive both RF and power line signals. Has a real time clock and memory to hold downloaded timers and simple macros. It can run with the computer off.
IMHO the CM15A is the only choice I would make.