The low-fat craze has ebbed somewhat in recent years as other nutrients -- primarily carbohydrates and, in particular, sugars -- have become the new villains of the diet scene. But many foods are still marketed with the claim that they are "fat free" or "low fat."
Whether a mix or a homemade cake rates better on the tasty-scale, will probably be a debate that goes on, well, forever. But from a health standpoint, there's no debate. Homemade cakes are healthier, even if it is dessert.