Aromasin binds permanently to the aromatase enzyme. The others bind temporarily. Your body makes more enzymes each day thus you need to dose accordingly. You can get away with less aromasin, than letrozole or arimidex.
In the end it's a question of how high your AS dose and what type.
I had the best results w/ letrozole when my test levels were a gram/ week. Closer to the show I will use aromasin.