If you're shooting a photosynth to get the most out of it, it's almost the opposite of shooting a pano.
Photosynth does very well when you pick a subject and then walk all the way around it taking photos of it from all sides. If you can walk all the way around something in a complete circle, then 30 or 40 photos is a good number to take. When you do this, Photosynth is able to keep track of little details and watch how they move in relation to each other to figure out how far they are from each other in 3D. By using the [P] key on your keyboard, you can toggle between photos only, photos with point cloud behind them, and point cloud only. If you shoot enough coverage of an object, the point cloud begins to look a bit like a 3D model of the subject.
Photosynth doesn't need you to move in a circle, but it's a good way to learn. Remember, Photosynth can only make a point cloud out of the parts of the photos that overlap each other.