Tamifxifen blocks the estrogen receptor in the breast, whereas aromatase inhibitors block an enzyme called aromatase and keep if from converting androgen into estrogen. (Postmenopausal women
get most of their estrogen from that conversion process, while premenopausal women
get most of their estrogen from their ovaries.)
I can't speak as to anyone's experience with Aromasin (I'm sure others with post about that!), but, of course, side effects very from indivdual to individual.