Most people seem to have ball locks as they were more readily available and they have a smaller footprint in a keezer. You cant go wrong with either, but i would recommend sticking to one type or the other just from a "spare parts" perspective. I like the smaller diameter ball lock, but pin lock folks like the fact that you can't mistakenly put the wrong connector on the wrong post.
I believe more people go with ball locks because they are slightly taller and skinnier, making them easier to fit into a freezer or mini fridge ( why I chose ball lock) 8.5"x25" for ball lock vs 9"x23" for pin lock. Pin locks have a twisting connection with I believe 3 threads, where as the ball lock has the same adaption mechanism as air hose quik disconnects. There are several threads on this, I would measure what you intend to put them in before purchase
Traditionally, ball locks have a manual pressure relief in the lid making it easier to purge the keg of O2. Pin locks typically have a mechanical relief making purging more difficult but not impossible.
Yeah, atleast he didn't ask do you prefer pin lick or ball lick. That could be awkward.
I am just now about to keg two batches of beer for the first time and I went with ball locks. Mainly because the first keg I ended up with in a trade is ball lock and I wanted to keep everything the same so the next one I bought I stayed with ball lock. As everyone has mentioned, there's pros and cons to both. I'm not sure if one is vastly superior to the other.