PCM is better in that it will detect a loss of signal and tell the servos to remain the same for a while. FM gets a bad signal for a few milliseconds and passes this bad info along to the servos. So the servos glitch. When both are working, there is really no difference. But if the signal is weak, PCM will appear to be better. Some would argue that they would rather see the glitches that FM passes along. PCM can mask intermittent failures of signal.

And PCM is a form of FM. Both are frequency modulated, but PCM layers a digital encoding on the fm transmission.