[Lani]

the answer is not in yet

But at San Antonio this upcoming December there will be a lot of poster discussions about it:
Aromatase Inhibitors
101 Factors predicting survival after neoadjuvant therapy with aromatase inhibitors.
Murray E, Renshaw L, Macaskill EJ, Murray J, McCaig F, Young O, Cameron D, Kerr G, Thomas JS, Jack S, Dixon JM. Western General Hospital, Edinburgh, Scotland, United Kingdom.
102 The benefits of letrozole in postmenopausal women with early stage breast cancer who have had five years of tamoxifen are independent of age.
Muss HB, Tu D, Ingle JN, Martino S, Robert NJ, Pater JL, Whelan T, Palmer MJ, Piccart MJ, Shepherd LE, Pritchard KI, He Z, Goss PE. University of Vermont Cancer Center, Burlington, VT; Queens Universtiy, Kingston, ON, Canada; Mayo Clinic Cancer Center, Rochester, MN; University of Southern California, Los Angeles, CA; Fairfax Hospital, Fairfax, VA; McMaster University, Hamilton, ON, Canada; Jules Bordet Cancer Institute, Brussels, Belgium; University of Toronto, Toronto, ON, Canada; Massachusetts General Hospital, Boston, MA.
103 Letrozole suppresses tissue and plasma estradiol, estrone and estrone sulfate more effectively compared to anastrozole.
Geisler J, Ekse D, Helle H, Eystein Lønning P. Haukeland University Hospital, Bergen, Norway.
104 A detailed analysis of the benefits of anastrozole over tamoxifen for venous thromboembolic events (VTEs) after 5 years' treatment.
Cuzick J, Wale C, on Behalf of the ATAC Trialists' Group. Wolfson Institute of Preventative Medicine, London, United Kingdom.
105 Anastrozole and letrozole an investigation and comparison of quality of life, tolerability and morbidity.
Dixon JM, Renshaw L, Young O, Murray J, Macaskill EJ, McHugh M, Dixon OM, Folkerd E, McCaig F, Cameron D, Dowsett M, Langridge C, A'Hern R, Fallowfield LJ. Western General Hospital, Edinburgh, United Kingdom; University of Sussex, United Kingdom; Royal Marsden Hospital, London, United Kingdom.
106 Letrozole-induced arthralgia is not consistent with an autoimmune disease resembling Sjögren's syndrome: preliminary results of a multicentric prospective trial.
Azria D, Lamy P-J, Belkacémi Y, Roux C, Gourgou S, Ozsahin M, Zaman K, Llacer Moscardo C, Lemanski C, Gutowski M, Boneterre J, Romieu G, Gligorov J. CRLC Val D'Aurelle-Paul Lamarque, Montpellier, France; CRLC Oscar Lambret and Lille II University, Lille, France; AP-HP CHU Cochin, Paris, France; Centre Hospitalier Universitaire Vaudois, Lausanne, Switzerland; AP-HP CHU Tenon, Paris, France.
107 An integrated analysis of zoledronic acid (ZA) for prevention of aromatase inhibitor associated bone loss (AIBL) in postmenopausal women (PMW) with early breast cancer (BCa) receiving adjuvant letrozole (LET).
Brufsky A, Bundred N, Coleman R, Lambert-Falls R, Mena R, Dong M, Schenk N, Lacerna L, Perez E. Novartis Pharmaceuticals Corporation, East Hanover, NJ.
108 Mechanistic studies of aromatase inhibitor (AI) resistance.
Chen S, Wang X, Masri S, Phung S, Wu X, Yuan Y-C. Beckman Research Inst. of the City of Hope, Duarte, CA.
109 Trastuzumab improves the responsiveness of letrozole refractory human breast cancer cells to endocrine therapy.
Sabnis GJ, Brodie AM. University of Maryland, Baltimore; UM Greenebaum Cancer Center, Baltimore, MD.

I have already seen papers trying to compare efficacy and side-effects but
there doesn't seem to be a consensus yet.