I'm sure others will chime who have actual experience with this, but here's my .02.

You do not need to run a PCM RX with a gas engine. By the way, be sure your radio is PCM capable if you decide to go that route.

PCM is rumored to be less susceptible to interference than PPM, but a properly set up gas engine installation will cause you no more trouble than a glow installation.

Then you get into the PCM v.s. PPM debate. The way I see it, PCM's advantage is that you set which way the control surfaces and engine speed will go if it loses a signal. I think the main advantage is that you can set the engine speed. Certain factors affect what the plane will do v.s. how you have the control surfaces programmed so that's a little iffy to me, but knowing your engine will go to idle (for instance) if you lose a signal, at least, assures you won't have as far to walk to pick up the pieces.