Hi AnalogJ,
You can see some of the different properties of different types of wood here. The rated bending strength of poplar and spruce are almost identical.
The overall construction of each foundation including the width and thickness of the slats and the gaps between them would also make a difference. A quilted surface on the foundation would also be a little easier on the mattress ticking in terms of wear and tear than if a mattress is resting more directly on a harder wood surface. If you call Sleepmaster they would probably be happy to provide you with the details you need about the WJ Southard foundation.
There is more about foundations in the foundation post here but the end result you are looking for is a support surface that flexes as little as possible under weight (you can check for any flex yourself) and provides for an evenly supportive surface under the mattress. Slats that are closer together have less risk of any sagging in between the slats.
As far as appearance that of course would be a personal preference
Phoenix