the 9" has an extra bearing/support on the snout of the pinion, making for a stronger interface between the ring & Pinion, and a larger diameter ring gear. more aftermarket parts BY FAR, easily weldable formed plate steel center section, opposed to the cast center of a 12 bolt. easy gear changes, cheaper LSD's, etc. blow up a 9" third on a road trip, slap another one in from a junkyard, blow up a 12 bolt on a road trip, find a new rear.
