photo diode and photo transistor

what is the difference between a photo diode and photo transistor????????

i have read about them but could'nt figured it how we would come to know to either use photo diode or go for photo transistor?????????

are there any hard and fast rules or it only depends upon the designer of the circuit to choose any one of them to fulfill his or her purpose????

how we would be able to justify that the choice made by us to use photo diode or photo transistor is a good one or a bad one????????

 

Here is what Wikipedia has to say - http://en.wikipedia.org/wiki/Photodiodes.

Designer usually have a body of experience in the field of electronics that helps them selects devices to use in different applications.

Performance of the circuit and obvious improvements would be the criteria to evaluate choosing a photo device of one kind over another.