Area13, there are a lot of posts from various people in different forums that say that the difference is valving. At least, that's what the say "people" at Bilstein tell them.
Isn't your truck approximately stock height if not lower in the rear once you add air? Mine is.
Currently, and I say currently because it seems Bilstein changes their site.
ALL BILSTEIN B6 4600. FEATURES AND BENEFITS
Optimum grip and enhanced lane stability in day to day and extreme situations
High gas pressure and a longer service life even when pulling trailers or driving off-road
Enhanced safety
No spring change required (Use of OE spring possible)
BILSTEIN monotube gas-pressure technology
Individually fine-tuned in the BILSTEIN road test
Digressive valving instantly reacts to changing road surface conditions for maximum comfort and control on and off-road
Durable vulcanized neoprene rubber bushings
ALL BILSTEIN B8 5100. FEATURES AND BENEFITS
46mm monotube design ensures consistent, fade-free performance in all conditions
Patented digressive valving instantly reacts to changing surface conditions
OEM certified mounting hardware allows for easy installation
Zinc plated for resistance against harsh elements
Durable vulcanized neoprene rubber bushings
Available in various lift heights for multiple applications
Made in the USA or Germany