Ok. Soy has isoflavones, which are kinda shaped like estrogens, and bind to estrogen receptors by chance cause they have similar structures, but not as strongly as real estrogen, because they're not sex hormones to begin with.
Cow milk has actual estrogen. Because cows are mammals. With actual sex hormones. Which binds strongly to estrogen receptors, because that's what they're for.
I know which one I'm going with.