I like to use a combination of the two. Buttermilk gives these waffles the best flavor, while milk helps produce a lighter texture. The purpose of buttermilk in most recipes, including waffles, is to add acid to the reaction with baking soda to make it “rise” higher. The thickness helps the dough retain the air pockets that acid and baking soda create when heat is applied, resulting in a “lighter and fluffier” waffle.
Buttermilk waffles are tastier, but the batter is thick and the waffles are less crispy. Waffles made with milk, on the other hand, are crispier but less flavorful than buttermilk waffles. A combination of the two milks offers the best of both: milk for a crunchy texture and buttermilk for a full flavor. Buttermilk is also a complicated ingredient because it tends to make a heavier dough, resulting in waffles that are less crispy.
Next, the acidity of the buttermilk helps the baking soda work even better to create a light, fluffy waffle.