Are thumb-operated or finger-operated trackballs better?

It depends on the work. Across the reviewers we read, thumb-operated trackballs like the Logitech Ergo M575/M575S are praised as the easiest transition from a traditional mouse and the most comfortable for all-day office use, since your wrist stays planted. Finger-operated, large-ball designs like the Kensington Expert are favored by reviewers for precision work and for distributing effort across multiple fingers, though they have a steeper learning curve.