I'm in the process of creating a study for my company on the feasibility of switching from PLC based controls to and MCU controls
In my opinion and experience the determing factors are
1) Does the end-user change program - if yes PLC, forget 2)
2) is this unit always running the same program - if yes, uC, if no PLC

As to your question those that can be answered, the answer simply is "think quality"

Erik