RE: Question about Radio's
First off understand that the transmitter is the same and will transmit both PPM (fm) and PCM. The model designation only refers to the reciever that ships with the package and this reciever accounts for the price difference as well.
Both are transmitted on an FM signal, its only different in how the information is encoded and how its handled by the reciever.
PCM is better at rejecting interference from adjacent channels and noisey systems like gas engines with electronic ignition.
Lockout sends the reciever into fail safe mode where the servos either stay put or assume a pre programmed position, most commonly used to neutralize the control surfaces and bring the throttle to idle.
If you can afford the difference, get the PCM. If your just starting out and want to save a little upfront get FM and if you get something later on that you feel you need PCM for just purchase a PCM reciever, the difference is set in a menu in your transmitter.