Western U.S. Peppermint is regarded as higher quality than non-U.S. Peppermints because it has a smoother, more refined odor. This is due to several minor constituents being at slightly different levels.
 
Western U.S. Peppermint has a higher menthol content (desirable, as this is the main therapeutic constituent) and a lower pulegone content (also desirable, as this is a less pleasant constituent).