Hi Celia,
That must be so confusing to get different opinions from different specialists. Perhaps I can clarify at least one apparent misunderstanding. Aromatase inhibitors (AIs) are a class of drugs that stop the production of estrogen in postmenopausal women. There are several kinds, for example:
Anastrozole (Arimidex)
Letrozole (Femara)
Exemestane (Aromasin)

It sounds like the first oncologist suggests anastrozole (brand name Arimidex) and the second prescribed exemestane (brand name Aromasin). Studies have found that they are equally effective, but they have different side effects. Every person reacts differently, so the choice is usually made according to which side effects you experience and which you can tolerate.

Tamoxifen is the oldest and most-prescribed selective estrogen receptor modulator (SERM). While an aromatase inhibitor is the first hormonal therapy medicine choice for postmenopausal women, tamoxifen is the first choice for premenopausal women and is still a good choice for postmenopausal women who can't take an aromatase inhibitor.

A number of studies have compared aromatase inhibitors with tamoxifen to see which type of medicine was more effective in treating early-stage, hormone-receptor-positive breast cancer in postmenopausal women.

I copied selected information above from the website BreastCancer.org from these 2 articles:
- https://www.breastcancer.org/treatment/hormonal/aromatase_inhibitors
- https://www.breastcancer.org/treatment/hormonal/serms/tamoxifen

I hope this helps to give you more information to discuss with your oncologist. I also look forward to hearing the experiences of others who may have had to choose between an AI and Tamoxifen.