In the kitchen and bathroom you don't need one. You don't sleep or rest there, but are alert and would probably notice smoke or a problem.
Many smoke detectors are not suitable for bathrooms. But it's good that you reminded me that I still wanted to buy a heat detector for the bathroom since we now have a washing machine and dryer there.
Depending on the size of the kitchen, I would choose a smoke or heat detector. A smoke detector is more likely to cause false alarms if you forget to turn on the extractor hood or if the kitchen is quite small and the smoke detector is close to the stove or dishwasher.
By the way, we have a heat detector in the kitchen. It wouldn't help with a burnt pizza, but of course it would if there was a fire.