Richard,

I guess it depends on the complexity of the project.
Pro for PLCs
1. PLCs are really good at sequential and repetitive tasks.

2. PLCs generally are hardened against EMI, ESD and other external environmental factors.

3. PLCs have modularity.

4. Off the shelf components, fast turn around on design.

You could design a MCU based board keeping in mind to design it so that it is modular, EMI protected, ESD protected, etc.

Cons for PLCs
1. Not very good at complex calculations.

2. A/D and D/A is usually slow, not good for fast moving waveforms,etc.

3. Packaging is usually larger than MCU based design.