There is a good run down on 12s and 9s from jason at the GMR on my thread here
https://lateral-g.net/forums/show...t=37889&page=2
If you already have a 12bolt, I certainly wouldnt worry about it. Not a great deal in it really.
The only real advantage of the 9" that cant easily be replicated into a 12bolt is the easily interchangeable third member allowing easy track side ratio changes.
Amit