why would you even want to reduce estrogen to such a degree? not only is that unhealthy in itself, letrozole is also hard on cholesterol levels, possibly carcinogenic and then there's the joint pain that comes with it.
i wouldn't touch an AI except to deal with a case of gyno flare up. still, mast can prevent gyno and it has other, nice benefits as well. costs as much as a solid AI from a research company anyways.