Aromasin is a suicidal Aromatase inhibitor, which makes it a better choice than other AIs. Exemestane binds with aromatase enzymes and then subsequently permanently disables the enzyme by destroying it.
Aromasin is a steroidal AI which keeps it from negatively affecting your lipids like other non-steroidal AIs. It is also not liver toxic, and can be ran for longer lengths of time without negative effect. This makes it a better option to run on cycle from a pure health standpoint. Aromasin will not hurt your gains on cycle like other AIs, but may actually help them. Another benefit on cycle is that steroidal AIs lower SHBG which increases the ratio of free to bound testosterone. What that means during your cycle is a very positive impact on your gains. This (in a sense) makes your Anabolic Androgenic Steroids more bio-available.