ah here we go, someone was bound to have written a whole thesis on it somewhere on the 'net ... Crystals are made of quartz and use the pietzo-electric effect. A piece of quartz will vibrate or oscillate when a voltage is applied across it and conversely will give out a voltage if it is flexed or vibrated mechanically. Pietzo cigarette lighters operate by flexing a crystal and then amplifying the voltage generated to form a spark.
I know pyro as a type of electric wire/flex that is cocooned in a metal sheath packed with powder - you have to have special equipment to fit the "fittings" to it.
It withstands fire and is/was used in military installations and factory/minning applications.
Anyways - back to the thread , did I say "Get the Piezo?"
Go for the piezo! Upgraded tops & matching headstocks are for looks -not tone. You will have so many more tonal options in one guitar, and if you play it a lot, the top may get dinged anyways. When people see my SM-Y2D standard w/Floyd their jaws drop. I didn't see the point in spending the extra money. I put it towards my Luke w/piezo. These two instruments can cover just about anything.