If your wife has an ER/PR+ type cancer, I would go with the tamoxifen. It has an excellent 20+ year history of preventing recurrence.
I took tamoxifen for the recommended five years, and then switched to Femara. They did bone density studies first, because Femara affects bone density, and my studies were super good. Even though I took the extra Vitamin D and calcium supplements along with Femara that first year, when my 2nd bone density study was done, my studies had gone down, down down. After consulting with my oncologist, it was decided that the couple of percentage points Femara gave me to prevent recurrence were outweighed by the 100% damage the Femara had done to my bones. I stopped taking it. That was about 6 years ago.
My diagnosis sounds worse than your wife's. I'm a little beyond 12 years from diagnosis and treatment, and so far, Thank God, no recurrence.
Tell your wife to ask about the potential side effects of both drugs. Depending upon her age, that also may give more guidance into her choice.
Good luck and best wishes.