Other than what has been mentioned, it is my understanding that both work just fine. There is the size difference already mentioned but also the keg coupler on the Pin Locks is taller so while the keg is shorter the overall tapped height is about the same and you are stuck with the bigger footprint with the pinlock. May or may not be an issue depending on your fridge. That is why I think the ball lock is in more demand which in turn increases the price.
The big thing is that whatever you go with, you want to stick with it. It is a pain to have to switch between pin lock and ball lock couplers all the time. Though a pin lock keg can be converted to a ball lock coupler to keep that uniform. I guess that would mean that you could go the other way as well. I am new to this as well so don't quote me on that last conversion bit.