When I got into kegging, I found a better deal with a bunch of ball locks, thus I went with them. If the reverse was true, I'd be a pin lock guy.
FWIW, there is a pressure release valve on ball lock kegs, I'm pretty sure it doesn't exist with pin locks, but there are easy work-arounds. I just find the built-in valve useful for purging out O2.
There's nothing wrong with either. The only downfall is that the pin's usually don't have a pressure release valve as someone else mentioned.
The only one thing (to me) is that ball-locks are taller and thinner, whereas pin locks are slightly shorter and fatter. My kegarator can hold 6 ball locks but it wouldn't be able to hold 6 pin locks due to the wider size.