Soy is a complete (all the essential amino acids for growth) source of protein.
It's not prefered because many people believe it can act as estrogen in the human body. I personally am mixed but have eaten a bit of soy my entire life and have most of a beard, extremely deep voice, and gain muscle very easily @ 19. So I doubt that to an extent.
I wouldn't recommend loading up on it though.
The best protein sources are real food. lean cuts of meat, seafood, dairy, eggs etc
Whey is a fast absorbing protein source. Hence why it's used post workout.
Casein which like whey is also derived from milk is a slow disgesting protein and is best used @ bedtime. Since you'll be on an 8 hour fast.
Unless you're vegan, and then mix in hemp protein and you'll have to combine protein sources, stick with whey and casein mostly.
Lifting one globe at a time!
Rep me + and I will return the favor.