Slate is by far the most forgiving and versatile, making it a great choice for the beginner and pro as well. Slate offers amazing sound quality, but is the most vulnerable to moisture.
Glass and Crystal offer a sharper sound than slate, making it great for cuts and cackles. While it is more weather resistant than slate, it requires frequent conditioning for optimal sound.
Aluminum has a very bright and nasally sound. Aluminum can be described as more "finicky" than glass or slate, but can accomplish any sound that a turkey makes, especially realistic kee-kee runs. Excellent weather resistance and low maintenance.
Copper is in a class all by itself. Copper is ultra raspy and nasally. It is effective in some cases as a locator because it hits a certain pitch that none of the others do. It is a blast to run, but would not recommend for a true beginner.
Conditioning your call...
Slate should be occasionally scuffed lightly with a scotch-brite pad. Avoid touching the surface, due to oils from your skin soaking into the surface.
Glass and Crystal should be scuffed with 80-120 grit sandpaper perpendicular to the direction of your striker motion. The heavier grit will produce a raspier sound. It is recommended to recondition each time you begin a hunt and after every few calling sessions.
Anodized aluminum should never be conditioned with sandpaper. Should it ever "slick over", clean it thoroughly with an alcohol wipe. You may lightly buff it with scotch-brite, only if necessary.