Both Pt100 RTDs and Thermocouples have their advantages and disadvantages. The main advantages of an RTD Sensor is that they are very accurate over a wide range and combined with excellent stability, this is in comparison to a thermocouple which is less accurate, less stable and are known to drift over time.
RTD Sensors are more suitable for lower temperature ranges, where higher accuracy and repeatability is required. They also need to be immersed in the media due to stem conduction errors.