When should I use proportional control instead of on-off control?
Proportional control will always provide more accurate control of your process, but the controller and metering pumps will be a little more expensive. Here are some guidelines to determine if you can save the money or not.

On/ off control can generally be used for applications where the response time of the system to chemical additions is fast, and/or where the acceptable range of the pH/ORP is wide. In a small, well mixed tank, on/off control can maintain the pH within 0.1 or so of the set point. In a well designed industrial wastewater pretreatment system, where there is sufficient residence time of the water flowing through the tank, and the acceptable level of pH may be ± 0.5 or more, on/off control will also be fine.

Use proportional control whenever there is an extremely tight control range (± 0.1 or less), where there are long delays between adding the chemical and detecting the addition, or when the set point is at an inflection point in the titration curve such that when a small addition is made the pH changes dramatically.