Nutritional value of proteins may differ substantially depending on their (essential) amino acid composition and digestibility.
Whey Protein is a dairy based protein and is rapidly digested. Amino acid composition of whey proteins is very similar to that of skeletal muscle hence they can efficiently promote muscle protein synthesis.
There are 20 amino acids, and your body needs all of them to maintain muscle mass and strength. However, 8 of them cannot be synthesized in the body and therefore need to be consumed.
Soy Protein is termed an 'intermediate release protein', releasing amino acids into the blood a little slower than whey but higher than casein.
Whey Protein is a fast-release protein, that gets digested rapidly and leads to a fast release of amino acids into the blood.