Look at your suppliers, and find out how many have ball-lock vs pin-lock kegs available. think about whether or not you might want to buy some more in 5-10 years. Ask them which they expect to be more available.
Ball-lock seem to be far more prevalent in the suppliers that I've seen.
Also: You can take apart a ball lock keg with a standard deep-well socket. You need special tools for pin-lock.