As far as the choice between the two in terms of effectiveness, they are both good (and superior to Aromasin in preventing recurrences other than bone mets).
Femara is also reported to have less effect on cognitive functioning than
tamoxifen, for example, and I don't know if that holds true for Arimidex as well. Tamoxifen is the one with some risk for uterine cancer. I don't recall seeing that caution for the AI's but I could be wrong.