Ok....I decided on the Logitech Trackman Marble FX. I've owned several Logitech trackballs in the past, and loved them all.
Incidently, I did order a Logitech Trackman Marble FX at the end of last week, but the one I received feels sort of "gritty" as compared to my two Marble Mice (all optical trackballs). I think the problem is with one of the rollers...I'm not sure. I just know that the Marble Mouse that I used prior to this (as well as my husband's Marble Mouse) moves as smoothly as glass. I'm chalking this one up to a defect, and returning it. Since I needed to order another trackball, I thought about taking a look at the MS Trackball Explorer, but in the end, decided to go with another Logitech Trackman FX. Other than the "gritty" feeling of the *defective* FX, I really loved the thing. Hopefully the replacement will run more smoothly. 🙂
Kalliopy