At work I use a Logitech TrackMan Wheel and I have a Space Navigator, but I'm not sure I'd equate the two. Are you wanting a new trackball only for SketchUp or for other programs too? I've never tried to use a 3D mouse for anything other than SU or Google Earth so I'm not sure if or how intuitively it will work in other programs. I could never get the hang of SketchUp with a the Navigator; maybe I don't use it properly. It seems that I couldn't do everything with the Nav that I could do with my trackball and it took my hand away from the keyboard so shortcuts became slower to use. The Navigator or one of 3dConnexion's higher-end products would probably be better for your carpal tunnel than a thumb-driven trackball though.