The difference between spearmint and peppermint is that peppermint has a bold and strong flavor that can be overpowering in recipes; spearmint has a sweeter and more mild flavor than peppermint. The difference also lies in the way that the leaves are grown; spearmint leaves tend to be bigger and lighter in color than peppermint leaves.
Peppermint is a more common mint when used in candies, sweets and lozenges. It can help balance out the sweetness in these foods and is a way to enhance the flavor. Peppermint also contains menthol, which causes a chemical reaction in the mouth that makes the mouth think it is colder than what it truly is. Peppermint can give the mouth a feeling similar to when spicy foods are eaten.
Spearmint is more ideal for people who do not enjoy bold flavors and who do not want the effects that menthol has on the mouth. It provides a flavor that is similar to that of peppermint and provides the same freshness that is found in peppermint without providing the overpowering cooling effects that peppermint has. Whether spearmint or peppermint is used in recipes, in gum or eaten as candies is dependent on personal preferences.