What will make your stomach flatter? whole wheat or whole grain?

Both: When choosing carbohydrates like bread, rice, rolls, and even cookies, choosing whole grains (including whole wheat) will help improve your health and reduce the development if a big belly. The less processed a food is generally the healthier. So a baked potato is healthier than mashed or fries. Brown rice is healthier than white. Whole grain bread is better than white.