For specific circumstances, one is better than the other. They are both fantastic instruments and can make a variety of sounds. Depending on what day it is, either one may work better than the other. For early morning hunts, many veteran callers prefer slate. This is because it can still be used in spring when the air is particularly dry and glass surfaces are susceptible to moisture. Glass calls can be louder and more pronounced if the goal is to call a faraway bird during the day.