Historically, PLC's were considered better suited for discrete logic and smaller i/o counts while DCS's were better for analog signals and larger i/o counts. While there are MANY reasons to consider either, now days they are fairly comparable in ability. As to programming, the PLC has different "languages" that may be used such as ladder logic, structured text etc, and may be easier to program without seeking formal training. While the DCS not only has the processing function programming it also has the integrated "operator interface/SCADA" programming which may be out of the comfort zone for a programmer that has not had DCS exposure. There are many considerations as to whether one can replace the other so it is best to do a comparable cost/effectiveness analysis to determine which system is better suited for which operation/plant.