They are functionally similar in the plant with regards to pathways, but chemically phosphite is more soluble s easier to get in to the plant. Once there it needs to go through a conversion process that can take time. Many sources claim a fungicidal benefit, which I suspect is due to it being reducing (it is missing one oxygen relative to phosphate). We have not used it in the soil, but I suspect it has the same conversion process, which may or may not include shutting down mycorrhizal fungi. Likely cost-prohibitive to use as a soil application anyway.