Dual conversion gives divides the incoming signal twice for better noise rejection of certain types of interference.

With PCM you need a transmitter that is both PCM AND the same manufacturer, you cant run JR pcm recievers with Futaba or any other transmitters besides JR, the encoding is proprietary.

PCM is the way to go if you already have a transmitter, if not then 2.4ghz is the hot ticket right now.