The main difference is the connections to the keg. Pin locks have pins on the posts that the connectors twist and lock into. Ball locks have a sleeve with little ball bearings to connect to the posts. Pin locks were Coca-Cola and ball locks were Pepsi/everybody else. Ball locks are easier to find generally. There is a slight difference in the dimensions of the kegs themselves, but it isn't much. I would check around with area home brewers to find out what the prevailing type is in your area.
Drinking on the keg: BPA, Brown Ale, Dry Mead, Wee Heavy aged on Oak, CAP
Drinking in the Bottle: