While both baking powder and baking soda are leavening agents, they have different properties. Baking soda is stronger and requires an acidic ingredient to activate, such as brown sugar or buttermilk. Baking powder, on the other hand, contains both an acid and a base, and it is activated by moisture.
In most cookie recipes, baking soda is the preferred choice. However, if you don't have baking soda on hand, you can use baking powder instead. Keep in mind that using baking powder may result in slightly different texture and taste, so it's always best to follow the recipe for the best results.