Hi Sudhindra
Both are good evaluation boards. The main difference is that R41Z evaluates the Rigado module that is the complete RF portion of the system in a PCB that you "plug" to your design. FRDM-KW41Z will allow you to do hardware modifications to the RF circuit or more flexibility in the circuit location on PCB, but once you design your product you will need to implement the RF part in your PCB considering RF best practices.
So the decision will depend on what do you need for your application.
Regards
Santiago