Aromasin would be 12.5 every day. I take 25mg eod just because I happened to get a great price on the 25mg tabs otherwise I'd be doing 12.5mg ed. I havent noticed a difference taking it eod.
arimidex and aromasin both work but I've read that arimidex affects your lipids in a negative way...eg, higher cholesterol..etc If cholesterol is not a concern for you then just take the arimidex.