The pH monitor will only tell you what the pH is. The monitor/controller which attaches to the co2 bottle via Solenoid valve, will not only monitor or tell you what the pH reading is, but will also help control the amount of co2 going to the reactor.
I actually have both. I use the monitor alone to keep an eye on the effluent ph level and adjust the bubble counter accordingly. I also have an aquacontroller II set up so that when the PH of the tank drops below 8.3 it shuts off the CO2.
You could use just a montior and switch it back and forth between the tank and the effluent and shut off the CO2 manually when tank ph is to low. The problem being is that PH usually drops at night when your not around to shut off the CO2 - and a calcium reactor tends to drive tank PH down, thats why I also run a Kalk reactor.
I agree with Tom 100% on using Pinpoints, whether you get just a monitor or controller or both.
I have a Pinpoint PH monitor and the Aquacontroller II. The Pinpoint is steady as a rock and although I love the Aquacontroller, it's PH probe is a little finiky, needs lots of attention.