Dave, the 2.4 RF link uses its own transmission scheme. The data stream is transmitted in low energy to the Tx antenna that will translate it to its own "language". So choosing PPM or PCM in terms of what is better is irrelevant.
However, the following can be said:

The PPM/PCM choice will influence the number of channel available, depending on the radio maker.
For example, the JR 10x will get 9ch in PPM and 10ch in PCM.

In PCM there are 25 source frames, whereas in PPM there are 50 source frames. So the PPM mode will be more precise ( if you can feel the difference between a signal that is updated 50 times per second instead of 25 times ). However the PCM mode will be more robust since the PPM transmission will have 50 redundant frames, whereas the PCM one will have 75 redundant frames.