Spearmint is lighter and sweeter than peppermint because its menthol content is minute compare to peppermint's. Menthol is a very strong constituent of peppermint which gives its strong scent and cooling feeling over skin. My experience is that although peppermint seems stronger to start with the scent does not last as long as spearmint in the soap, also I think that we can use larger amounts of spearmint as it has so little menthol as compare to peppermint, menthol may irritate skin in large amounts. I love both oils but for soaping my favorite is spearmint, hope this helps.