They are very different tools. UFOStretch is for modifying subsets of characters by scaling and interpolating on a single axis. It is useful for tasks like creating small caps and figures. Superpolator is a multi-axis interpolation tool that's mostly useful for interpolating and extrapolating entire fonts from an arbitrary number of master fonts.
Does anyone have a preference between Superpolator or UFOstretch? Does anyone use both? Just curious, looking into both right now.