with more splines on the axle the axle has to be a larger diameter, which gives more strength to the axle. Ford I beleive has 28 spline and 31 in their more performance oriented cars.
For aftermarket axles, the housing will take up to 40 spline axles, but you need the matching spool.
To run a 35 or 40 spline spool in a ford, the carrier has to be able to use 3.25 bearings.
The weak point in a ford 9" is the pinion support tends to break. The N case is a little stronger, but if you intend to put a lot of hp and torque through it, get an aftermarket carrier