A built 12 inch is stronger than a factory 9 inch
The only way a 9 inch is strong is if you invest some money into it.
The more splines an axle has usually it is also larger in diameter. Larger if all else is equal is stronger.
Very few cars can be powerful enough to twist top quality 35 spline axles. I have seen above 3,000 hp on Mark Williams 35 spline with no isues. I have also see a couple of other brands damaged with 35 splines with 1,600 hp (appx) in addition to tire shake.
I prefer Mark Williams but they cost more money.
If you do a 9 inch Ford be sure to get a strong aftermarket third member case, and an aluminum pinion bearing retainer.
Ed