I have both the Logitech Trackman wired versions--the one with the thumb and the one with the fingers. My preference is for the thumb and here's why.
With the thumb model, my hand is resting naturally, even when idle. The thumb is resting normally too. When in use, only the parts of my hand that have to move, have to move, so the amount of effort/strain is minimal. I can work 12hrs+ on one of these with zero strain (just like my IBM M).
On the finger model, my fingers must rest extended on the ball--not terribly strained in any way, but after about 6hrs, I feel tendon strain on the top of my wrist. Also, fingers are most comfortable moving in one axis--bending. The finger ball requires them to move sideways too, which also adds to the strain.