It's pretty common mistake due to their mechanism of actions. Exemesane (Aromasin) basically "kills" the aromatase enzyme while Anastrozole (Adex) merely binds to the enzyme and keeps it from exerting its effect.
But just like the difference between a hand grenade (kills some) and a nuclear bomb (kills all), Aromasin is dose dependent so if you are at a reasonable dose you can easily manage your estrogen and keep it at an optimal level. I like it better than adex, because even though the half life of the drug itself is less than adex, its effects actually last longer since the killed of aromatase has to basically be created again before it can do anything, making it longer lasting than Adex.
Plus adex kills my libido and it hums along nicely while on Aromasin!