Pin locks and ball locks have different disconnects.
Standard ball locks are slightly skinnier, slightly taller. 25" tall by 8.5" diameter.
Standard pin locks are slightly fatter, slightly shorter. 23" tall by 9" diameter.
Pin lock disconnects, however, are significantly taller than ball lock disconnects, so the height advantage of pin-lock kegs are pretty much negated by this.
Pin locks often don't have a pressure relief valve built into them, whereas ball locks do, which is a nice thing to have.
Pin locks seem to be more available these days, since most of the ball lock kegs have gotten snapped up by homebrewers.
New ball locks are being made, however. I don't think anybody is making new pin locks.
Cut out a bunch of 9" diameter circles from a piece of cardboard and lay them out in your keezer. If it looks like you might be able to squeeze one more keg into your keezer if you had 8.5" circles, then go with the ball locks.