A chef knife (or gyuto) and a santoku are both general-purpose knives — but they answer different questions. A chef knife has a belly that lets you rock through a pile of herbs. A santoku is shorter, flatter, and made for up-and-down cuts on compact board work.
If you cook one big mise en place for a family, the chef knife is usually the right answer. If you cook shorter, faster prep and want a lighter blade, the santoku earns its spot. They are not better or worse. They are two jobs.