A chef’s knife has a curved belly and a tip. You rock, you slice, you use the point. A santoku is flatter, often with a sheep’s foot tip — push-cut, chop, little rocking.
If your week is roast chicken, herbs, and the odd joint, the chef’s knife is the one. If your board is mostly vegetables and you already push-cut, a santoku will feel like it was waiting for you.
You do not need both. Two similar 18-centimetre blades in the same drawer is how blocks get full and knives get dull.