A 12 bolt looks to be cheaper, but why? I hear 9 incher are "beefier" internally. and for a 450 maybe a little shy of that with a 125 shot would I anything with one of those rearends and a 4000+ rpm lauch on some good drag radials?

30 spline or bigger?

 

a 9 inch is stronger but takes a little more power to turn sence its so big and it weights a little more than a 12 bolt does, I went 9 inch on mine sence I have a TH400 with a trans brake I know its going to leave hard and I dont want to brake parts.

 

The 9 inch has more parasitic drive drive loss due to the poor pinion angle..not the actual size ring and pinnion. The Dana 60 is actually bigger than the 9 inch...but has less drive train loss then a 12 bolt due to a better pinion angle for the fbody.

Cons
The 9 inch will not work with 3 channel ABS.
it is heavier than a 12 bolt.
Has more parasitic drive drain loss.
It was designed by Ford.

Pros
The center section is removable which makes changing gears, or rebuilding the rear a breeze.
It is much stronger then a 12 bolt and can be made to handle as much power as you could ever throw at it.