The Nikon SP is better built internally, and has a rock-solid shutter (basically the same shutter as the one in the Nikon F). A tank.
But the lens choice will be limited. You're limited to the Nikon lenses, and in theory the Contax lenses (and Kiev RF russian lenses) will mechanically fit just fine, but they will need slight modification of focus distance to be able to focus sharply with the Nikon.
There are great lenses for the Nikon RF system (and very expensive btw), but far more choice for the Leica M system. If you want choice, the answer is clear -- Leica. But if you want a really unique object, the Nikon SP is the dream rangefinder of many collectors.