When mass matters, comparing stiffness or strength alone can favor a dense material that produces a heavy component. Specific properties divide a mechanical property by density $\rho$.

The specific stiffness is

$$\frac{E}{\rho},$$

and the specific strength is commonly represented by

$$\frac{\sigma_y}{\rho}$$

or by tensile strength over density, depending on the failure criterion.

Simple comparison

Suppose material A has $E=210,\mathrm{GPa}$ and $\rho=7800,\mathrm{kg/m^3}$, while material B has $E=70,\mathrm{GPa}$ and $\rho=2700,\mathrm{kg/m^3}$.

Then

$$\frac{E_A}{\rho_A}\approx26.9\times10^6,\mathrm{m^2/s^2},$$

$$\frac{E_B}{\rho_B}\approx25.9\times10^6,\mathrm{m^2/s^2}.$$

Although A is three times stiffer in absolute modulus, the two materials have similar stiffness per unit density.

Specific properties are useful first comparisons, but they are not universal design rankings. Geometry, loading and the design objective determine which combination of properties actually controls minimum mass or another performance target. Those combinations are derived as material performance indices.