The topic came up another forum that I read so I asked over there why whey over soy and this was the answer

The BV rating for whey is among the highest of any protein source. Bilogical value is a rating system based on absorbtion and utilization.

The BV ratings:
Whey = 104
Egg = 100
Casienate = 91
Beef = 80
Soy = 74

So for the money (economics vs. efficiency in assimilation) you can see which have the best bang for the buck if one is spending money for supplemental proteins. One could spend as much for a trendy soy protein and yet not have the same yield, however, some have to pick the source based on any food allergies or eating principles such as certain vegetarians may not desire a source derived from an animal.

If you'd like to view the thread or ask questions over there its:
http://www.cyclingforums.com/showthr...82#post2967782