Dear Shobha
The trial below only compared Evista to Tamoxifen as a bc preventative in high risk women but it prevented cancer head to head against Tamoxifen with far less side effects.
A nine year bc survivor at my local support group took Evista instead of Tamoxifen because she had a blood clotting problem (another condition) and couldn't take Tamoxifen so the onc tried this.
I think the 2 drugs will give similar results in preventing mets or recurrences its just the studies aren't out there yet. But results in preventation are and the 2 drugs act the same.
Secondly, you would never be permitted to take both drugs at the same time.
I hope this helps and eases your mind as you ARE doing something about that estrogen receptor after all.
http://www.cancer.gov/STAR