The suitability of a thermocouple or RTD Pt100 sensor will depend on the application and measurement requirements of the user. It is incorrect to say that one is better than the other as both thermocouples and Pt100s have their own merits and uses.
However, we can compare the advantages and disadvantages of each type of sensor to determine which type of sensor would be most suied to a particular application:
Temperature Range: Thermocouples have a wider temperature range than Pt100's with some able to measure temperatures up to 1600ºC and beyond. For most industrial applications Pt100s are limited to 600ºC and more often to 250ºC.
Accuracy: If high accuracy is of most concern, then a Pt100 is the better choice in most cases. Even the least accurate Pt100 (Class B) will generally be more accurate than a thermoocouple. Pt100s are available with very high accuracies, 1/10 DIN elements, for example, have an accuracy of ±0.03ºC at 0ºC.
Stability: Thermocouples tend to drift over time due to chemical changes such as oxidation whereas measurements from RTD Pt100 sensors are very stable and repeatable.
Response Times: Whilst ever smaller diameters have improved Pt100 response times, thermocouples, especially with grounded or exposed junctions, have an incredibly fast response to changes in temperature.
Durability: Because of the more fragile nature of Pt100 elements, thermocouples are considered the most rugged and durable of the two. Especially for high vibration applications where Pt100 sensors are not suitable. A thermocouple is a simple rugged sensor that can withstand significantly more mechanical abuse than a Pt100.
Cost: Thermocouples are generally less expensive to manufacture and have lower material costs, except for the cabling where thermocouple cables are slightly more expensive than the copper cables used for RTD sensors.