I have a simple recommendation. If you choose PCM, use a glitch detector. That increases your advance warning of interference issues just as on PPM, but you also get the advantages of failsafe and error correction.

I don't fly PCM without a glitch detector if I can help it. And I fly my expensive sutff on either PPM or PCM without worrying about either. They both work well in our application, and in the end its the failsafe capabilities that lean me towards using PCM over PPM. That way I can decide what I want to happen if it looses the signal completely, rather than having no control at all.

Other than that, I have never had problems with PPM over PCM regarding interference. Even when going for eye challenging distance limits.