pros and cons of a 9 3/4 rearend

I do not have one. But if your going to get one, I would guess that you'll ethier need to have it custom fit or allready purchased that way. The only ones stock for the car that I can think off may be the HEMI cars from the factory from the late 60's.

The Pro of it would be that the rear is a strong as they get.
The only Con I can think off is spending the bucks on a OE or custom fit.
The power cunsumtion factor I think is a overrated thing. It may consume more power to turn it, but it ain't jack IMO.
I have seen a few TV tests on this topic. The amount of extra power from small to large rear was nothing really.