I use both. At work where I have a small desk, and uncomfortable ergonomics, I use a trackball. It's just better because I can move it into any position that is comfortable and still have "mousing" ability. I wouldn't have that luxury if I had to have ample room to track a mouse.
At home though, I prefer an optical mouse. Games just aren't as comfortable with a trackball as they are with a mouse. You have a greater margin of error with a mouse than you do with a trackball. By this, I mean that a movement on a trackball has greater effect on the cursor than a movement on a mouse. Of course, all of these movements can be tweaked, but generally speaking, my logitech trackman marble is generally "faster" than my logitech optical mouse.
It's just a comfort thing I guess.