Soy can lower estrogen. This has been proven. In fact, the reason it's recommended for women is because it helps prevent cancer by it's anti-estrogenic properties.
I have one scoop a day.........soy isolate with my oats.....whey in after training and a slow-digeting (casein usually) in bed...........gotta have that mix, the body becomes accustomed way too easily.........
Both are fine. I use whey myself, its cheap, available, and high-quality. The phytoestrogens in soy stick to the receptors in your bones much better than the receptors elsewhere; meaning you shouldnt get tits from using it. I wouldn't go overboard with the soy just to be on the safe side. Soy generally causes a larger increase in IGF-1, however (I believe most plant proteins do).