Bishops and Priest, whats the difference?

I have both Priest and Bishops, but I build them all where I have a master guild that makes my Priest more powerful. So my question is, what is the difference between the two and how to I get one instead of the other? I really didn't pay attention to my build order in my capital as I figured I was going to use it for the College of Cardinals training facility and so far it works nicely for that.

Bishop is an extra +1 piety. You get a bishop instead of a priest when you train the new cleric at a city that has a cathedral. You don't get it from a huge cathedral in vanilla, due to an error in the VnV file, but that's an easy fix. It'll probably be taken care of with the upcoming patch, Carl's problem fixer has it included, and it's a simple text edit if you have neither of these. The trait Bishop gets replaced by the trait Cardinal if the priest gets promoted, so you don't get an extra +1 at that point.