Concur with Nelson. The legit versions used for treating women are 1mg tabs of arimidex or 25mg tabs of aromasin. So, as far as that goes, they should be almost equal. However, aromasin is a suicide inhibitor, permanently binds the receptor site on aromatase enzyme, Arimidex doesn't permanently bind, so I'd tend to think that makes aromasin a tad stronger. B/c once those receptors are blocked, they stay blocked until the body kills and recycles that enzyme. Arimidex, the enzyme could be blocked, but come unblocked and still be active, if only for a little while before another molecule of arimidex blocked it up again. Basically, that's the long-winded, I'm bored on sunday morning before the gym answer... they're pretty much equal as nelson said, I'd tend to think aromasin might work a little better though.