why a Ford 9"?

Aftermarket support is not real limited on either unit anymore.
If you have an 8.75" already, I can't see it being much of a price difference in building one or the other...ya might even come out cheaper with the 8.75". It's not like you have to do much in the way of beefing it up...unlike the ford. Like it or not, believe it or not...the 8.75" IS stronger.

The spline count is not that big a deal either. If you are wanting it to be real strong, you will probably be adding either a spool or a locker and upgrading your axles. In that case, you have your choice...35 spline axles are readilly available.

If you are just buying one ready to run outright, the 9" will be cheaper.
Someone mentioned the difference in shaft size and the weight. The difference in shaft size is not where the extra strength comes form, it's the bigger bearings. The only difference the weight is gonna make is in handling (if any). It may actually help to have a heavier unit, since we are all nose heavy anyway...LOL
Performance-wise, the weight of the rear overall and the weight of the rotating assembly have virtually no effect. The difference in weight is not enough to make a difference in acceleration. This was tested a while back. Same car ran a few passes with an 8.75" and then they pulled it out and bolted in a Dana 60...ran the same numbers. I can't remember how much heavier the 60 is, but it outweighs both the 8.75" and the 9" by quite a bit.

Do your research and run what ya want...it's your car. Like I said earlier, I won't run a ford rear in my mopars.
Am I the only one that's sick of seeing a small block chevy in virtually everything? Boring azz motor! LOL