PCM advantages:
Servo movements without glitch, even if the model is far way.
Holding of the servo position during short glitches (Hold).
Moving the servo to a predefined position in case of a longer disturbance or
even complete failure of the transmitter (Fail-Safe).
Servos are not damaged by pulses that are too long/short, which could happen
with PPM.

PCM disadvantages:
More expensive.
Sensitivity to adjacent channels is usually worse comparing with PPM receivers.
Care has to be taken when flying near to a transmitter from an adjacent channel.
Due to different protocols, only receivers from the same brand or even type of
the transmitter can be used.
Checking the transmission quality can be difficult, because the hold-mode
smoothes out small glitches.
The lack of early warning signs may cause trouble.
Control problems that build up gradually, e.g. of a technical nature, get noticed
only when the connection fails completely, which may lead to a crash.

PPM advantages:
The PPM system is cheaper.
There should be no problems using different brands of receivers with different
transmitter manufacturers.
Transmission is fast enough to operate even the quickest of servos.
With PPM, the end of the transmission range is shown by the servos starting
to glitch. When the pilot notices this, he/she can probably still get the model
back home safely.

PPM disadvantages:
Due to its simplicity, PPM system cannot detect errors, the receiver does not
see the difference between valid and invalid servo pulses. When the range
boundaries are reached, pulses get slightly longer or shorter because of noise.
Servos start to glitch. This may happen when antenna orientation is not optimal,
when the projection of the receiver antenna is nearly down to a single point, the
signal breaks down and the servos get false pulses.
These short glitches go unnoticed most of the time because they are smoothed
out by the servo's and the model's inertia (response time).