I believe the key difference is that a photoresistor is basically a resistor whose resistance is a function of the incident light, while a phototransistor is a transistor whose gain is controlled by the intensity of incident light. You could duplicate a phototransistor by using a photoresistor and a normal transistor (at least according to the minimal research I've just done on this topic in the past two minutes).
Another key difference seems to be that photoresistors have a much lower response time than photodiodes and phototransistors (which can be orders of magnitude faster, according to wikipedia). Apparently this means that you want to use phototransistors for high-speed applications like data links and picture scanning. I think you want to use photoresistors for things like color detection as you can get better analog measurements than with phototransistors, which are more suited to on/off type results.
- Ben
