Contour mouse has a thumb scroll wheel, and a wheel that you can rock back and forth. They all can be programmed very easily with their software. Thumb wheels are so much better, as your index finger doesn't have to move around and thumb is the strongest finger, which fit the repetitive scrolling movement better.
The main advantage however is they offer 5 sizes for each hand, ie. xs, s, m, l, xl for the right handed, and s, m, l for the left handed.