I can't stand the MS version, Logitech makes one (TrackMan Marble) that I have had since the first iteration. I currently own 5 of the current rev. I sold my original model on ebay for $30 about a year ago, mind you it was PS/2 ONLY. I have no idea how I lucked out on that one, but you might luck out as well.
I think the buttons are too far forward of the trackball on the MS one... It would be really nice if Logitech updated theirs with more buttons, horizontal scroll, etc...