From what I've learned it seems that Whey is much better then Soy. The BV value is higher and BCAAs are way up there also. Then you take into account protein utilization and it makes perfect sense to use Whey instead of Soy. But if your looking to stay away from animal products then Soy is it for you. I think that you will find that most body builders prefer Whey.
For years, casein has been the "gold standard" of protein quality, and most experimental researchers still prepare casein-based diets for their animal studies. But as it turns out, whey protein is much superior to casein in terms of protein quality. There are various techniques for measuring protein quality but suffice it to say that four methods are commonly used: protein efficiency ratio (PER), biological value (BV), net protein utilization (NPU), and chemical score. Regardless of which technique is used, whey always scores better than does casein, as shown in Table 1. Whey is also far superior to soy and other lower quality proteins such as rice, beef, and wheat (which are similar to soy protein).
Whey and egg are, for the most part, comparable, and are the two best dietary sources of protein. Whey has a higher BV than does egg, which is important, since BV reflects actual protein utilization in adults. In contrast, PER is a measure of the growth rate of young animals, which is less pertinent to the situation in adults (the higher PER of egg is due to its higher content of sulfur-containing amino acids, which are especially important during growth). From the standpoint of exercise physiology, the main difference between whey and egg is that whey contains higher levels of BCAAs, which makes whey a better protein source for athletes and bodybuilders.
A COMPARISON OF WHEY, CASEIN, EGG, AND SOY 1'2
There are three important attributes of whey protein which make it an ideal source of protein for athletes and body-builders. First, whey is very high quality protein, and is hence important in maintaining muscle structure and function during periods of strenuous exertion. High levels of high quality protein exert an "anti-catabolic" effect, meaning that muscle breakdown is prevented.
In addition, whey protein is one of the richest sources of BCAAs, which play a central role in muscular function. BCAAs (notably leucine) are known to be directly oxidized in muscle, thereby acting as a source of energy. In addition, leucine is used for alanine synthesis in muscle; alanine is subsequently transported to the liver and used to make glucose, which is then available as a fuel for muscular contraction. The high content of BCAAs in whey strongly support its use by athletes and bodybuilders alike.
Finally, recent studies have shown that lactalbumin (the main protein in whey) can enhance immunity in animals. In one study by Bounous and colleagues, mice were fed diets containing various proteins at a level of 20 percent.3 After eight weeks of dietary treatment, the animal's immune system was assessed using a standard assay (the plaque-forming cell response in spleen). Lactalbumin proved to be 2.4 times better than egg protein, and five times better than casein with regard to immunoenhancement! The stimulatory effect of lactalbumin was apparent at two weeks and persisted throughout the course of the study. More importantly, the immuno-stimulating effect was observed when lactalbumin was replaced by an amino acid mixture duplicating its amino acid pattern. This firmly demonstrates that the amino acid composition of lactalbumin is responsible for its enhancing effects in the immune system. The significance here is that strenuous exercise has recently been linked to a depression in both the cellular and humoral immune responses of adult humans.4 Hence, athletes and bodybuilders alike might be wise to use whey (lactalbumin) as their source of dietary protein.