The simple answer between AM and FM is AM send the signal by amplitude modulation (Remember the Reception and interference on AM Radios) and FM uses Frequency modulation. I was always told that FM sets are less prone to interference and therefore are a better choice. The Crystals are I believe used to tune to a specific frequency and are required for the radio to work. I have also been told that on most Radios you cannot switch freqs by changing just the crystals without sending the radio back to the factory.

I'm sure their are many out their who can provide a more scientific explanation


Just my 2 cents