Btw, the "failsafe" stuff Charlie was talking about is a function of the receiver, not the transmitter. In the past, if you wanted a receiver to throw out bad data and eventually go to a default if it lost signal, you needed PCM. However, now there are PPM receivers that have all the same features. Some are smart enough to recognize bad frames and hide momentary interference as well as PCM receivers and have programable fail safe settings. The programming is different, since it's done through the RX instead of through the TX, but the result is the same.
With PPM, you can use multiple brands of receivers, since it's an industry standard (just watch out for the "shift", or use auto-shift-sensing receivers).
With PCM, every brand transmitter has it's own way of encoding, so you have to use that brand receiver.
Basic 1024 PCM is pretty much the same as PPM as far as data transmission rates are concerned, and the signal between the receiver and servos is always the same regardless of PCM vs PPM, Digital vs non-Digital servos. If it wasn't the same, the gear wouldn't work in mix-and-match setups as it does now.
However, there ARE some transmitters that have modes that have special faster frame reates in PCM mode.
PPM transmitters/receivers are also limited to 9 channels. If you want more than 9, you have to be in PCM mode. And up until recently there were very few 9 channel PPM receivers out there, they mostly stopped at 8. So if you are doing a lot of mixing, or have a complicated scale bird that uses a ton of channels, you'll be forced to PCM for that reason alone.
This site has some great details about how our radios work. It's worth a read:
http://www.torreypinesgulls.org/Radios.htm