Why not take the money you would spend on soy and buy some real food. Like beef. It has a superior amino acid profile and ZINC which you may be deficient in. Good luck.
I plan on eating both, but was interested supplementing a small portion (about 1/4 to 1/3) of my daily protein needs. Soy protein seems to be the least expensive and very affordable. It would be 1,555 grams of protein for $24.
"Animal research suggests some great advantages of using soy protein isolate as a bodybuilding supplement. Research has shown that the isoflavone daidzein found in soy-protein isolate might have a gender specific normalizing effect on sex-hormone production. Lab animals experienced testosterone and growth hormone excretion as well as muscle growth in males, while the female animals experienced a decrease in these hormones and fat loss."
Soy Vs. Whey
"The latest studies into soy protein suggest it does not decrease testosterone and raise estrogen, as previously thought."
"... the American College of Sports Medicine (ACSM) Meeting, May 2004, and sponsored by the US National Dairy Council, found the post-workout consumption of isonitrogenous and isoenergetic soy drink to be statistically significant in hastening mass, fat-bone free mass (FBFM) and increases in strength..."
Synergistic: The sum is greater than the individual parts.
Synergistic Example: 2 + 2 = 5
What research shows is that combining two proteins with different amino acid profiles, magnifies the effetiveness.
"Used together in an exercise regimen, soy and whey proteins complement each other well. Whey protein, as mentioned, is high in branched chain amino acids, used as an important energy source by the body during exercise, while soy protein has high amounts of the amino acids arginine and glutamine."
The best thing to do is review the information posted above, do your own research and come to you own conclusions.
As for me, Soy Isolate appears to have some definite strength. Even better is the low cost of it.
Soy protein is cheap because its awful. Its also not a particularly good choice for males. Its a phyto-estrogen. Just do some research. Beef, chicken, fish, and eggs are great and supplement with whey or a whey egg protein blend. All of those give more bang for the buck than soy and taste better.