Hi Beth2014,
You can see the properties of different types of wood here (just scroll down). As you can see Spruce has a higher bending strength and stiffness than all three types of pine but the stiffness of a foundation will also depend on its construction not just on the type of wood.
When I talked with one of the owners of USBoxspring they told me it was made of Spruce (see post #295 here).
Phoenix