Third/Fourth-gen application, nodular iron third-member, disk brake flange application. What's the approximate weight difference if both are loaded and ready to bolt up?

about 45 lbs. yes i have had both assemblies in my garage and just for the hell of it i weighed both with out brake assm. on housings, and the 9" was just a stock center not a nodular. i run a 9" in mine and with stock suspension set-up the car went,,,, well you know. and i dont mind the extra weight in the rear of the car because it was scaled and all 4 tires are within 5 lbs. of each other. no bs... perfect 50/50 weight distribution. talk to ya later.

My Moser 9" with 3:89's, locker, nodular iron center section, 35 spline axles complete without the brakes on it weighed in at 217lbs. That thing outweighs me by close to 50lbs I think a complete 12bolt weighs in the area of 170lbs from what I've heard.

So why is a 9-inch so popular when a 12-bolt weighs so much less, and is more efficient at transferring torque, and has a pinion that isn't offset ?

I'm curious because I only want to buy a new rear axle once.

9" is stronger and easier to change the ring and pinion. But a 12-bolt is plenty strong for 99.99% of 4th gens out there. So for the reasons you cited (and the ease of preserving ABS) that's why I run one.

BTW: thanks for the weights, that's useful info!

Rich