Which is best and why?

Whey, by far. Whey is highly bioavailable, so you absorb and use more of it than you can of soy. It has a similar amino profile to our own muscle, over 75% BCAAs. Soy is the bottom of the list for protein. The hierarchy goes like this:

1. Whey

2 and 3. a tie between egg and casien

4. Animal protien (chicken, fish, beef)

5. Soy.

there was some recent research b/w soy and a negative effect on sperm. too much conflicting information on soy for me to ever consider it

personally, i'd put casein and egg over whey

Well, no worries with the sperm. I've been cut LOL

o.0 lol wtf

A 2004 study presented by the American College of Sports Medicine showed that soy is as effective as whey for muscle growth, and that in the study's 41 male athletes, there was no increase in estrogen, and no decrease in testosterone.

well, there have been studies that say soy is fine, and studies that say it reduces test.

but, everyone can agree on the benefits of whey protein.

to me it seems all protein's have there place, for soy it's place is for people who are vegitarians or can't eat animal protein's. In the future it might be used to partally replace animal proteins maby up to 50% to help conserve the earth. anywho soy or plant protein is harder for your body to use than animal's.
I also agree with abear that casein and egg provide a more positive effect on the body than whey, for general all around use. this topic can get prity in depth so ill leave it at that.