The Stumpjumper has a higher end component mix and, more importantly, the frame is an aluminum boron composite. This makes the aluminum harder and stronger. You can make a lighter frame without loosing strength. The Rockhopper uses another kind of aluminum which is strong enough but it has to be thicker to reach the same strength so the frame is heavier. Both are good bikes but the Stumpy is just a little (or a lot, depends on how you look at it ) better. Plus it's legendary.