i bet you'll get a hunderd replies on this one but here goes
i've been flying about 20 plus yrs. i fly giant scale 1/4,1/3 & up
helicopters and faaaast prop planes.this is what i do to determine which i fly i use both fm (ppm) & pcm. first if if the motor has spark ignition no question its going to be pcm too easy
to get interfernce from the plug firing. helicopter too many moving
parts metal to metal.glow power planes have no problem in fact most every plane i have that is glow is fm never had a problem.
now if i'm going to fly a oh say a $1000.00 plane even if its glow at this piont the few extra bucks isn't going to matter.pcm has the advantage when your getting interfernce(provided you don't program in fail safe) it will hold its last comand it will also let you punch thru some signal control surfaces will be half as fast as normal but you will have something(better then nothing) fm when it grets hit can act like a wild squirrel running around and the reset time is longer so its possible if the interfernce doesn't stop long enough you proberly won't regain control.the only thing i would suggest is you might consider useing fail safe on the throttle only when it goes in fail safe the motor goes to a idiel.
you might want to install a fm first in perhaphs a gas plane to check for interfernce(you can see it with pcm but you have to know what to look for)its easier to see with fm ithen fix the problem althou i have seen some slight problems that could not be resolved with out pcm.of course there is one other thing to consider the first pcm i bought was well over $200.00 now just about a hunderd so price doesn't have to be the only detemaning factor.we had a couple of guys in our club flying gas planes with fm and said they never had a problem until the day they went in.there is a big advantage to pcm.also range check wil greatly improve i've gotten 3 to 4 times more on pcm but you need to weight out the options for your self.
now you will proberly get another 50 options this has been discused before.
roy