BloO, a proportional thermostat is always "on". it regulates the current going to your heat source to keep a constant temp. They can be programed to the 10th of a degree normally.

A "on/off" such as a ranco is exactly that, its basically full throttle then off until it cools down again to where you have it programed to be and it then turns back on.

I use rancos. My incubator is on a dual ranco and the reason is that if my power were to go out, when I tested the ranco vs a herpstat on my generator I had issues with the proportional. It would simply cycle on and off as it was not clean enough power to stay at the tenth of a degree it was programed to. The ranco is on or off, it did not really care about the power and ran perfectly on the generator.