Well maybe I wasn't looking at it wrong. This is from page 4 of Bilstein's PDF
http://www.bilsteinus.com/fileadmin/user_upload/user_upload_us/pdfs/Bil_LevShks_WebArticle.pdf
Increased Rebound Control
The lift on the 5100 Series leveling shocks is achieved by increasing the preload on the factory coil. By adding more preload, the shock is designed with more rebound damping as compared to the O.E. or standard aftermarket replacement shock.
So it sounds like they do increase the preload and then control that by adjusting the rebound, and increase the stroke length to restore travel.
If the Rancho QuickLift are well built then it seems like they would be a better option, assuming you wanted approx 2" of lift, because you can adjust them for your use and they are designed for the full height. It seems the Bilseins would have to get worse as you move them from whatever the optimal setting is, which is probably somewhere in the middle.
Sorry if this is a thread jack...