As long as it's steel, the material selection doesn't affect the stiffness of the rod. That's because the Modulus of Elasticity for all types of steel is pretty much the same, 28 - 30 Mpsi.

Where the material choice comes into play is whether the pushrod stays bent after it flexes or springs back to original shape. Low strength materials are easier to permanently deform than high strength materials, speaking only of steel varieties.

R.