Aromasin is superior in my opinion for most people. It's a suicidal inhibitor and lipid friendly, and lowers shbg. It's also stronger than arimadex.
I will say that for people like me, that have a very high tolerance for estrogen and are not remotely estrogen prone, I really have to watch my dosing with aromasin. I can crash it quick if I'm not dosing carefully. I have to use the mentality less is more for my own use