Nothing they are both FM.

The difference is the way the stick information is encoded and sent to the reciever. As for the price difference pcm recievers are more and thats why the pcm sets cost more, the software is the same. Are they better? Well thats debateable but they do offer you the opportunity to set fail safes in the event you loose signal. This would drive the controls to a preset position until signal is regained or the model hits the ground.