One is a SERM and one is an AI.
Having said that, I'm currently running 12.5mg ED of Exemestane (Aromasin), while on cycle. I'll continue to run it throughout my cycle, PCT (with Clomid also), and 2 weeks after.
I think most people keep Nolvadex on hand for Gyno mostly. I bought some, but decided not to use it.
-Pack