Generally a photo transistor is more sensitive to low levels of light than a photo diode.
And then we have photo Darlington transistors that are even more sensitive.
When you put them in a circuit you will need to adjust the load resistor to get the required voltage (actually a LOW voltage) when the light changes from darkness to the minimum amount of light to be detected.
You will find the load resistor is a low value for a photo diode and the highest current will be consumed by this device. A photo transistor will require a higher value load resistor and less current will be taken by the circuit and a Darlington detector will need a higher load resistance and the circuit will take the least current of the three.