hi,
An important point you should be aware of is the response of the circuit when you substitute a transistor for the JFET as shown in the original circuit.
If you are required to monitor the 200mA/20mA over a close tolerance the transistor response is OK, but if you need to monitor the current 200mA/20mA from say 0mA to maximum mA the the JFET is the correct solution.