I have used both Sidetrack and iScroll. I finally decided to use iScroll2. Sidetrack has a lot of nice features. You can hot corner the trackpad, set "touch and click" to different mouse buttons, and vertical/horizontal scroll along the sides. But it cost $15. While iScroll2 doesn't have as many features, it does allow you to scroll horizontally and vertically and the "touch and click" feature to the trackpad. Finally, iScroll2 is free. That is the main reason I use iScroll2 over Sidetrack.
Note: for those who don't know what I am talking about when I say "tough and click," these programs allow you to configure your the trackpad to when you hold you finger on the trackpad and click the trackpad button, a different button is registered depending on your settings (I had mine set up for right-click).