Traditionally, gin is used when making a French 75. In recent years bartenders have begun using cognac to add a modern twist to this cocktail classic. Cognac is distilled from wine and aged in French oak barrels. It has a fruitier, smoother flavor profile than gin which tastes more herbal and earthy. Gin tends to have more of an alcoholic taste, whereas cognac's milder flavors make it easier to drink straight up or on the rocks. For those who haven't tried either, I recommend you try both and see which you prefer.