I saw a test where they ran a twelve bolt, a nine inch, and a dana, all the same width, with stock drum brakes. They were all put under a chevelle to compare horsepower loss. In the process, they weighed all three. The dana was heaviest, but only by about 20 lbs. The incredibly heavy dana, just isnt. The dana also used up less power than the nine inch, but we all knew that.
Granted, the truck danas are heavier, because of thicker and sometimes larger axle tubes, but that just makes them stronger in a full race situation. Seen any nine inchers under a race car that didn't have a back brace lately?