I feel the number of fingers used in a gesture maps well to the 'scope' of the gesture, that's how I have BetterTouchTool set up. I use two-finger gestures for things like scrolling and zooming (content-level), three finger gestures for things like back, forward, next tab, previous tab (window- or app-level) and four fingers for expose/task switcher/hide all windows (multi-app/OS level).