Hi
Unless you enjoy keeping track of zillions of things, pick one or the other and stick with it. Duplicating connectors and repair parts is an expense as well as a hassle.
Some ball locks started out life as pin locks. That gets you into three categories rather than just two. If you go for ball locks, you may want to check to be sure they aren't converted pin locks. If they are they will be fatter (and shorter) than the rest of your ball lock kegs.
There is no real difference between the two, other than "real" ball locks should have a pressure relief valve on the lid. That can be convenient for burping kegs (dropping the pressure). It a *very* small difference, but about the only one. That pushes one in the direction of going with ball locks.
Bob