If your going to go to track ball to help/prevent Carpal tunnel then forget about the anything with a marble size ball it will just make things worse.

To help with CT you will need to get one with a large Ball. The only available track balls I have seen with a big ball and the one I use is a Kensington Expert Mouse. The ball size is about 2.25 inches
This is the unit that the HR department at my companies gives out if you have Carpal Tunnel issues. And it does really help a lot. Many of my coworkers have had Carpal Tunnel issues due to constant use of computers at work.

Resolution wise it works better then most mice, but it is limited to 4 buttons and a scroll ring.