The 31-spline axles are about 30% stronger than the 28-spline axles in torsion and run larger bearings (ID and OD) which make them stiffer for less deflection outside the bearing.
There are were two different axle diameters/bearing ID's used, Ø1.563 and Ø1.531 for SET20 and OE sealed ball bearings, though if you count the aftermarket sealed ball bearing options, those are available with either ID.