According to Men's Health (
http://www.menshealth.com/cda/articl...0-1307,00.html), while soy has some health benifits such as reduction in prostrate cancer and cholesterol levels in blood, soy phytoestrogens mimic normal estrogen and contain pharmacologically active molecules. These drugs have a negative effect on mental ability when ingested in large amounts. Keep your soy intake under 25 g per day.
Whey has all the health benifits of soy - redutction in prostrate cancer, lower cholesterol - and doesn't have any of it's side effects.