I had the chance to compare the WJ Southard foundation (they only make one as far as I’m aware) and the Green Sleep S-050 foundation in person. They’re both similar in that they both use a Canadian spruce frame and slats, and then it’s covered in organic cotton backed by wool. For what it’s worth, though, the WJ Southard has more slats, resulting in the slats sitting about 2" apart, while in the Green Sleep S-050, the slats sit about 3" apart. The outer fabric designs are a bit different, but in our situation, we have a bed skirt so it will be covered in either case. One other thing – WJ Southard will customize the height of the foundation up to a height of 10". The Green Sleep comes in one height - 7 3/4".
Are the number or distance between the slats enough of a difference to [i]make[i] a difference? Dunno.
I don’t know if this will be useful to anyone else, but at least this info is available.