A coarse spline is in fact stronger - I recall reading a service bulletin on the subject when Ford went to a coarse spline on the later SOS transmissions. They did the same on the input shaft of the later 8-speed crashboxes.
That said, lubrication makes a big difference. Notice how the splines are the same size on the other end of the shaft, and you never see any wear there. I always use a heavy duty spline lube on the torque limiting clutch splines and mating input shaft when putting a SOS trans back together.