How accurate is a photoresistor?
Cadmium Sulphide (CDS) photo resistors were commonly used in film cameras for determining exposure, usually wired in Wheatstone Bridge configuration. These light meters were quite sensitive and repeatable. Here's an example circuit:-



Since you only want to determine which light source is brightest, absolute accuracy is not important. With a Wheatstone bridge circuit you can adjust the potentiometer to get a null reading (equal voltages on each side) with one light source, then switch to another source and note whether the reading swings positive or negative. 

A photo resistor's value can change with temperature and aging, but this shouldn't be a problem so long as you do all comparative tests at the same time and don't let the light source heat up the sensor. At low light levels the response will be slow, so you may have to wait a few seconds for the reading to stabilize. 

The biggest problem you may have is different spectral outputs of your light sources combined with the response of your sensor. Incandescent light bulbs emit a broad spectrum that is skewed towards Infrared, while LEDs and Fluorescents have sharp peaks at certain colors. CDS sensors are most sensitive to green light. 

If you want to compare perceived brightness then you also have to consider the human eye's spectral response. CDS is a good match. Other sensor types can be quite different, leading to unexpected results.