There are a few key differences between Manhattan and Old Fashioned cocktails. The main difference is in the type of sweetener used.  For a Manhattan, use sweet vermouth, whereas, for an Old Fashioned, use sugar or simple syrup. This gives the Manhattan a slightly sweeter taste than the Old Fashioned.  Another difference is in the garnish. A Manhattan is typically garnished with a maraschino cherry, while an Old Fashioned is garnished with an orange slice or zest.  All of these factors contribute to the overall flavor of these two drinks, with the Manhattan being slightly sweeter than the Old Fashioned.