Hi Jen,
Cyathula has a much more limited range of effects in the body than Achyranthes, and was thus the herb that was used traditionally. Jing Tang may be using Cyathula because it is cheaper, but I think it's more important to use what the ancients use, as they were aware of both plants, but chose Achyranthes. That to me is meaningful, and explained by the broader range of effects of Achyranthes
Hope that helps!
Steve