I think it is largely a matter of what you like. I personally only own triacts, but that was mainly due to the fact that when I was buying gear there was an interruption in the supply of ball locks, so they just weren't available. Now I'm used to them, and they work well for me so I have not seen a reason to change. The only annoying thing I've found with them is that when opening them it is possible to start swinging the gate open before the collar is turned all the way. This results in the gate jamming very slightly open. It is no big deal, I just let go, the biner relocks and I open it again. It usually only happens when I am trying to open the biner too fast -- I am pushing on the gate and rotating the collar at the same time.
I've used the ball lock at the climbing wall, and it is okay. My one complaint with them would be the ball seems too small, especially when wearing gloves. In that respect, the triact is easier to use. If you haven't already, check out the \"Gear Rater\" section on this website -- a few people have already weighed in on the topic there. Plus, there are some lesser known secrets I've found there -- like the fact the SherrillTree carries a ball lock with a larger ball -- which might mitigate my complaint from above!