Diameter is more important to stiffness than wall thickness.

Assuming all are made of the same material and length, a .060” wall thickness 7/16” pushrod is stiffer than an .080” wall thickness 3/8” pushrod...... which is stiffer than a .100” wall thickness 5/16” pushrod.

Lobe design, rocker ratio, and spring force are also key elements in determining what is required for a given combo.