What Is The Difference Between Femara And Tamoxifen. My Chemo

What is the difference between Femara and tamoxifen. My chemo dr (i underwent a lumpectomy and radiation and am now cancer free) wants to put me on a preventative medication Femara for preventative as I am 18 in the oxytype dx?. I have gone on line to check out femara and it has a lot of side effects. Why not do tamoxifen versus femara. What are the pros and cons of each?