I have some ball locks. I'm not an expert, pretty new to kegging.
From what I've learned there are 2 main differences:
1) ball locks have a pressure relief valve in the top that can be used to easily burp or vent the keg when you want to purge air or co2. Pin locks don't (maybe some do, but not all).
2) different sizes. One is slightly taller and thinner, but I couldn't tell you which because I only have ball locks.
Ball locks used to be much more common, but because of demand by homebrewers I think the opposite is true now. The gas and beer fittings are not interchangeable, so you probably want to settle on one kind and stick with it for all your kegs.