If you can afford aromasin I would use aromasin however it is freakin expensive.
Femara is stronger then arimdex but that isn't always a good thing, both will have a negative affect on lipid profile with femara's being more pronounced, as stated above femare can have a terrible effect on libido.
IF you have the money use aromasin. If not then your choices are between femara and arimidex. I think when deciding between these two you have to look at your cycle (aromatisabel dosages) and how prone you are to estrogenic sides! For some bro's arimidex just doesn't cut and they have to use femara, for others arimdex works fine! You should also remember in the chance that you may develop gyno symptoms while using femara by adding nolvadex into the picture you will reduce the effectiveness of your letro by 35-40%!
Here is a study for you too look at:
tamox can decrease plasma levels of letrozole ...
Drug and hormone interactions of aromatase inhibitors.
Dowsett M.
Academic Department of Biochemistry, The Royal Marsden NHS Trust, London, UK.
The clinical development of aromatase inhibitors has been largely confined to postmenopausal breast cancer patients and strongly guided by pharmacological data. Comparative oestrogen suppression has been helpful in circumstances in which at least one of the comparitors has caused substantially non-maximal aromatase inhibition. However, the triazole inhibitors, letrozole and anastrozole, and the steroidal inhibitor, exemestane, all cause >95% inhibition. Comparisons between these drugs therefore require more sensitive approaches such as the direct measurement of enzyme activity by isotopic means. None of these three agents has significant effects on other endocrine pathways at its clinically applied doses. Pharmacokinetic analyses of the combination of tamoxifen and letrozole have revealed that these drugs interact, resulting in letrozole concentrations approximately 35-40% lower than when letrozole is used alone.