The robust user interface is the same regardless of it you have an MCU or PLC behind it. The MCU will likely cost 10% of what the PLC costs, and it won't require you to buy software that costs many thousand dollars to program it. Both solutions are industrially rated for temperature, shock, moisture, etc. So it really comes down to development costs. I can't estimate the hours of my time based on what you've told me, but I am going to assume that I could develop this for an MCU in a comparable about of time that a PLC engineer could. So unless PLC engineers make significantly less than I do, I am pretty confident that the MCU solution is cheaper, as it always has been for me.