The ballheads allow full freedom of movement so you can easily orient the camera and lock in position. The RRS BH-55 has the added bonus of a tension adjustment so you can maintain the ball in an adjustable state that can be still moved to a different position without loosening anything (hmmm, I hope that makes sense).
A pan/tilt head lets you move in one plan at a time, lock position and then move in the other plan.
Quality and (as Steve mentioned) load capacity is very important with a ballhead.
The other item that is well worth using with a tripod clamp is an L-bracket on the camera as this makes shifting the camera from portrait to landscape mode very simple.
The Really Right Stuff site has a lot of useful information. Naturally they reference their products but a lot of it is applicable to much more when selecting and using photographic equipment.

