Ultimately there isn't a real difference, just different fittings, so I'm not sure why people prefer one over the other. As evanmars noted, ball locks do seem to be easier to find.
One thing to keep in mind, once you start down the road with one type of keg, you need to stick with it since the fittings are not interchangeable and conversion is not really feasible having multiple styles means having to swap out your fittings which is a PITA. So if your first keg is ball lock, make sure all your kegs are ball lock; it will make your life a lot easier.