PS is a phospholipid. It enhances the myelin shealth that surrounds the nerve. It is very good for brain and neuron health. It has some anti-inflammatory properties. It is very different from soy. Soy may help some with PMS, and for others, it does not. I depends on the body type you have. If you have estrogen dominance, unfermented soy may help at first, but there is a general tendency to go worse later. If you dont have estrogen dominance but is in fact estrogen deficient, then soy can be useful for long term use. So first thing you need to do if you have PMS is to determine if you are estrogen dominant or deficient.
You can read more about estrogen dominant here for general info, but if you have a specific personal situation, then you can PM me.