May sound simple but whats the diff. between FM and PCM transmitters? Is there a + to either of them, over the other?

As far as I know, PCM is synthenized radio, that you won't need crystals for. That's the impression I got, not exactly sure though.

PCM is Pulse Code Modulation.

it runs on normal FM (PPM as airplane guys call it) frequencies (using crystals) but the signal is encoded digitally instead of an analog signal.

digital signals are easier to "pick out" and allow the RX to discard any bad signals it gets allowing for a more reliable radio link. the downside is the cost of the equipment is greater.

also PCM RXs usually have failsafe settings that you can program into the RX so that if signal is lost you can have the servos go to a preset position (like full brake and wheels straight)