I vote on getting the logitech. Just got one yesterday and I really think I am going to like it better than my CST trackball (which is about 3 times the price). Though, I have relatively small hands, and maybe that's why the logitech is so comfortable compared to the giant CST. I'll post some quick pictures of my hands on both.
I'd definitely go for a large ball trackball. I never could understand those thumb trackballs. They look very painful - constantly using your thumb with no option to change fingers. With the large trackballs, you can switch fingers and relax your hand.
Because of my small hands dilemma, I find that I am moving my hand far less on the logitech than with my CST. The large trackball requires me to lift my hand over the ball to reach the scroll wheel. (I do like the scroll wheel of the CST much better though, since it's a hyperscroll). The CST also takes a little more of a learning curve. You have to train your hand to lift off of the ball when you click to keep the cursor from moving while clicking. I still have to do the same for my logitech, but it's just slightly lifting my thumb instead.
Some pros to the larger trackballs are precision. It's much easier to control a large ball with your hands, rather than a small one with your thumb. But that's not to say it sucks using a thumb ball, or uncomfortable either.
Someone with larger hands though could have the complete opposite opinions on the situation, though.
In the end, the best bang for your buck would be the logitech. And not to mention, it's wireless! (which isn't a bad thing in this situation. Although, I have never tried the kensington mentioned in the second post.
