I normally use a pan-tilt head because I tend to spend quite a bit of time fine-tuning the composition of my photos, and the head makes it much easier to make precise incremental changes in the camera's position. One other point you may want to consider is that a pan head is virtually essential for panoramas.
However, a ball head is much more useful if you tend to move around a lot during your shoots and prefer not to spend too much time setting up each shot. In particular, it is well-suited for following the action in sports photography.
I daresay that the best option is to get both a pan-tilt head and a ball head. However, if you really have to choose just one, I would generally say that a ball head tends to be more versatile; nonetheless, you'll have to make the decision based on your own needs.