Joe and RSkrause hit the nail on the head I have basicly the same set up they do: Moser 12 bolt, 33 spline axles, eaton posi, 4:56's, c-clip elim, aluminum cover, BMR track pack & subframes, Hoosier 26 x 10's drag W/ matching skinnies, QA1's and air bags. Best 60': 1.51. I had the same problem with the bolts comming loose , but I solved it with lock tight red and an impact gun.
Not to get off the topic, but I just wanted to say that there is nothing sweeter that pulling the wheels 8-12" off the ground from a 20 mph roll. I absolutly love my drive train.

 

uh.. next time you should not let the bolts back out i use red loctite and lock washers..

for the orig poster-
nothing is bullet proof. I have broken my 12bolt a couple of times over the years, but then again.. I am hard on parts. And as RIch said, if you broke the posi.. it doesn't matter whether it was a 9" or a 12bolt.

I see a lot of people dont take up slack in there driveline, and other "rear end danger" things at the track.

Hell, i went 1.7s-1.8s and mid 11s ona stock rear with out problems.. just gotta drive it as rear end friend as you can..

(note, rear end friendly doesnt mean not hard.. I launched my 401 ci motor off the rev limiter just about)

 

Personally, I think you should go with the 9". And then send the 12 bolt to me