
As zouzi indicated, these two drugs are very similar in effectiveness, and there is some evidence that they both are superioir in some ways to a third AI, Aromisin:
"December 14, 2009 (San Antonio, Texas) — New results coming from 2 mature trials with exemestane (Aromasin, Pfizer) suggest that this aromatase inhibitor differs slightly from the other 2 available products, letrozole (Femara, Novartis) and anastrozole (Arimidex, AstraZeneca).