Since this appears to be a shool project, I'm not going to give a straight answer, but I will give you things to think about.
What does a PID controller do? What can't it do well? What does a PLC do? What is the basic function and control algorithm of Proportional-Integral-Derivative control? Can you put this in a PLC? If you do, how would it be better than a separate PID controller? What about information sharing and operator interface considerations?