I prefer the pseudo ceramic/ Sla-Tek surface used by Knight & Hale for their Ol' Yeller and Pocket Puppy calls.
I've owned slate, glass and aluminum and believe that the Sla-Tek combines the best qualities of each with less maintenance. Slate hates moisture, glass requires constant conditioning and aluminum is partial to certain strikers.
The Sla-Tek can whisper like slate or scream like aluminum with any striker and to my ear produces a crisper cutt than any of them. You won't find a more versatile ten dollar turkey call than the Pup. My brother-in-law swears by his his as well and many a gobbler has been seduced 'tween the two of us.