The answer is easy - IT DEPENDS.
In general, it is much easier to build a machine using a PLC.
For a one off machine, a PLC easily wins.
For a small number of machines, a PLC easily wins.
If you are going into a production run of thousands+, then it may be that a micro controller is better.
If you want your machine/applcication to fit into a match box, a micro controller is the only way to go (for now...).
Remember, a lot of PLC's use micro-controllers for their CPUs and in their I/O, so using a PLC will not actually get you away from using a micro controller.
Doug