PPM and PCM are both FM carrier wave.
PCM allows you to have presets in the event of loss of signal so you have some control over how you crash.
(i.e.: I want to be making a right turn at low throttle when I smack the earth or a tree).
I have heard that PCM is more resistant to interference, but it "masks" small amounts that a PPM (FM) would show and allow you to land knowing there was a small problem before it became a fatal problem. Probably hearsay spread by cheapos like me who cling to FM/PPM.
[link=http://www.natew.com/rcheli/frame_main.cgi/html.PCMvsPPM?PRINT=TRUE]PCM vs. PPM/FM[/link]
[link=http://rcsource.hobbypeople.net/hsjack/rcaradio.asp]AM vs. FM vs. PPM vs. PCM[/link]
My opinion is that if you use digital servos you will benefit from PCM and see a difference in performance. If you use analog (I guess that is the other style) servos you are just as well off with PPM.