I own and operate both types of kegs currently, and they both are very similar. The ball lock kegs are several inches taller and a little bit skinnier than the pin lock kegs. So if height is an issue in your kegerator, make sure to measure the height of the kegs before purchase. The actual connection of beer and gas lines is where the main difference is between the two. The ball lock definitely feel sturdier and seem like they would make a better connection to the posts, although I have never had an issue with the pin locks I own. Just my observation. The pin locks are generally $10-$15 cheaper per keg than ball lock, and the fitting for both are usually the same cost. Just need to look into your personal situation to see which work better for you. Good luck with the decision. I hope this helps.
Edit: almost forgot. Some pin lock keg lids do not have the purge valve that most ball locks come with. This is just a minor inconvenience seeing as u have to remove the gas connection and press on the poppet when purging co2. Just don't push the beer side! [emoji16]
Thanks. I've got an old BM-23, and eventually planning a quad-keg setup. I want to standardize to one or the other, and didn't want to deal with switching out fittings everytime I tap a keg! So, you personally prefer ball-lock due to it feeling sturdier on the connections... then you just may have made up my mind! Thanks!