Both the grinders do the same task at the end of the day - crush your weed into a fine powdery mesh which can be poured into a joint or mixed with tobacco and be smoked up! An automatic grinder does it without any physical effort from the stoner. A manual grinder requires the stoners to place the weed in it and manually twist and turn it so that the teeth splice through the weed. Automatic grinders tend to give a more precise effect with lesser effort but tend to be costly. Some stoners also find manual grinders more of a cathartic experience. It totally depends upon what you are more comfortable with!