If they’re the same price and readily available, you might want to opt for ball lock. Ball locks have a pressure relief valve on the caps, whereas pinlock typically do not. Ball lock also have a smaller diameter, so you might be able to squeeze a few more into a defined footprint. Of course, pin lock kegs are shorter, which may come in handy if there are height constraints. Finally, pin locks can accidentally “twist” off, but there’s no way to accidentally put a pin lock connector on the wrong post.
I started with 5 ball lock, but when I was able to find 16 cheap pin lock kegs, I made the switch and sold my ball locks. Really no big difference, but I do miss my PRVs a bit (I use a fingernail or screwdriver on the gas poppit to achieve the same result)