Having both would be a serious PITA. Don't do that.
Pin lock, I believe, stands a little taller than ball lock.
There are fewer of them out there, so you're less likely to come across a buddy who has a spare to throw at you or a spectactular deal that lets you jump up your collection on the cheap.
Also, you'll have fewer friends who can loan you parts. Additionally, if you go to a picnic or event, odds are there will be a ball lock system there, so you might not need to haul your gear over and can just use the one that is there. I guess the basic theme in that line of thought is 'check with your local fellow brewers and see what they use'.
Also in my experience it approaches impossible to make a mess with ball lock whereas drunken manipulation of pin lock kegs has resulted in the occasional squirt of coke all over the place (I've only dealt with pin locks that were being used to serve actual coke products). In seating the posts on a pin lock you open the valve. If you screw up trying to twist the pin lock into place you can squirt a bit of cokeout the keg before you push back down and make the seal again. I've never had that happen on a ball lock because you just let go of it when it is pushed down far enough.
