In addition to the above, there is a considerable difference in the way they are used. As mentioned by aolalde, the PTCs will work with a "trip relay", in that they allow the motor to run up to their trip point, then shut it down. That is it.
With RTDs on the other hand, a good RTD relay system can be programmed for responses at different temperature levels and provide warnings or alarms before you end up with an unscheduled shutdown. This can be very helpful in critical path equipment. In addition, Motor Thermal Management systems such as GE/Multilin relays will use the RTD inputs to bias the "thermal register" used to model the motor performance. This means that instead of calculating the motor temperature based just on current and time, the system compares it to actual values read from the RTDs in order to detect trouble spots that might have otherwise been missed.
The downside to this is that if it is a 20HP motor, it does not make much sense to use a $6000 relay on it! Hence the appeal of PTCs.
"Venditori de oleum-vipera non vigere excordis populi"