The enviromental controller will only controls temp and,humidity. While the greenhouse master controller does the same thing and monitors and controls co2 ppm.
The enviromental controller will control co2, but not monitor it. So basically the unit doesnt actually know the ppm of co2 in the room. Its basically a timer you set for the co2. Which you could do with a regular timer from walmart.
The gmc will actually monitor the co2 level so it will turn on and off automatically tk keep the room at the ppm you set it to.
If your using co2, you want the greenhouse master,controller because it will accurately control the co2. The digital enviroment controller will simply work as a timer and you may be using too much or too little co2.