Aromasin is a suicide or irreversable aromatase inhibitor, meaning that it destroys the aromatase enzyme. Arimidex is a competitive AI which means that it binds to aromatase and the enzymes that are bound cannot convert testosterone to estrogen.
Aromasin can do a more complete job of reducing estrogen conversion. However, this is only neccessary at very high doses of aromatizable gear (in my opinion).
Well put. IMO, arimidex is sufficient for most AS users. I've worked with many bodybuilders over the years who did well with arimidex, so this is nothing new.