The first factor that decides if you are going to use a Santoku or a Chef’s knife should be your personal preference. Do you feel more comfortable using a shorter and lighter knife or a bit longer and heavier one? In the first place, you should use the one which feels more enjoyable to work with.

The next thing that you should base your decision on is whether you are using the knife to process meat, fish, or vegetables. Because of the pointy tip, the chef’s knife might be a bit more useful for cutting meat around the bones, that is if you don’t have a boning knife at hand. On the other hand, a santoku is a bit more convenient for fish and vegetables.

Do you need to scoop-up the pieces you are cutting? When dicing onions or mincing garlic, it is convenient to use the blade of the knife to scoop-up and carry the processed food from the board to the pan or pot. In this case, you should use a Santoku.

These are just some basic guidelines. However, if you own both knives, you will learn to instinctively pick the one you feel it is more convenient just before you start cutting your cooking ingredients.