Spruce has a combination of low density, high Young's modulus and high modulus of rupture (compared to most other woods) that make it a superior wood for bracing. That is why it is used almost universally in wood-only (non-composite) bracing systems. Using pretty well anything else will result in some combination of a heavier, weaker, or less stiff top. The species of spruce doesn't matter a whole lot as stiffness tends to follow density in the same way for all species. Due to the "cube rule" effect (stiffness of a beam is proportional to its depth cubed) low density timber will generally result in a lighter structure for the same stiffness. Supporting analysis in the usual place.
Trevor Gore: Classical Guitar Design and Build