That is not how material properties work.
All steels, regardless of heat treat and the specific alloy, have essentially the same modulus of elasticity (stiffness). Therefore, the steel with the higher yield strength (300M for the purpose of this discussion) will actually twist further before yielding and taking a set than the steel with a lower yield strength (Hytuf). This linear elastic regime is where you want your axles to live because if you start yielding them, you will have a low-cycle fatigue failure. Ideally you want to be stressing your axles well below the yield point to avoid high-cycle fatigue failures.
Ductility describes the elongation of the material in the plastic regime of the stress-strain curve. This is how much the material will deform after it has yielded before rupture. This only comes into play after that material has already yielded (permanently deformed).
Toughness is a factor of both the strength of the material and the ductility. Think of toughness as the area under the stress-strain curve, and it represents how much energy it would take to rupture the material. Both 300M and Hytuf have high toughness, but 300M is stronger, and Hytuf is more ductile. Hytuf will yield (think twisted splines) more easily than 300M, but they would break under comparable shock loads. In a steady-state high-torque situation like a rock buggy with a bound up tire, the hytuf shaft could break and the 300M shaft not even yield.
In an axleshaft, it has essentially failed once it starts to yield, so generally the material with the higher yield stress is going to be stronger. Ductile materials have a more favorable failure mode than brittle materials since they don't experience sudden catastrophic failure, and are more tolerant of over-load situations.
Things don't even last as long as stock shafts
