Go with whichever you can find cheapest. I started out with a dozen free pinlocks, then someone gave me ten ball locks. Being cheap I didn't argue with the gift but I did have to make some alterations on my hose ends so I could easily switch between keg types.
Ball lock kegs are taller and narrower, pin locks are shorter but have a bigger diameter so the space you have in your kegerator can be an issue. One thing I love about my pinlock kegs is that it is not possible to put a gas connector on the beer post, but you can do that with a ball lock keg and when you do it's really hard to get it back off without destroying something.