There is a significant difference between a smoke detector and a heat detector. Because they have different functions, the combination of both detectors is highly recommended.
Heat detectors sound an alarm when the temperature in the room is approximately 58 degrees. This is very useful in the case of a smouldering fire, that generates a lot of heat. Smoke detectors will not give off an alarm signal yet, but a heat detector will. Smoke detectors are, on the other hand, ideal for faster developing fires, which do produce a lot of smoke.
This is why it's interesting to install both a smoke detector and a heat detector in high-risk areas. Some smoke detectors already have a built-in heat sensor in order to be able to detect both kinds of fires. Always avoid installing smoke detectors in places where there may be a lot of (water) vapour, dust or gases. In this case, only use a heat detector to prevent false alarms from the smoke detector.
In short: always use smoke and heat detectors together for optimal protection.