I like slate and glass surfaces both. I have calls with both types that seem to sound good to the turkeys anyway. Ray Eye says some of his best "Turkey Calls" sounded terrible to other hunters. I try and let the turkeys be the judge.
These Ozark Mountains Ain't High But The Hollers Sure Are Deep
Forgot to answer the question. I prefer a two sided call, with the top surface being crystal and a slate soundboard. I can get two distinct turkey sounds out of it, and the slate allows me to play softer for in close hunting