I was under the impression that API's Stress coat was more of a healing product than a water conditioner. Seachem has their own version of this called StressGuard, but to compare prime and stress coat is kinda like apples and oranges isn't it? I think Prime is more like API's Ammo lock or Tap water Conditioner. I could be wrong. I've tried to figure it out by reading thought all their products on the company websites, but there are so many its hard to keep straight, Especially API. I don't doubt the usefulness of API's products, matter of fact, I use their test kits exclusively, but it seems that they have multiple conditioners that overlap.
I use the Seachem products mainly because they are less confusing. There are fewer of them. I use Prime to condition the water for water changes, and StressGuard for adding new fish or when moving fish, or when the fish display an unusual symptom.