Only you can really make that choice but if it was me, Id go for mastectomy then baby, having a friend with the same brca gene and she went from stage 2 breast cancer to stage 4 in a matter of months, cancer is nasty and Id want that risk gone
Breast feeding versus an 80% cancer risk, Id give up breast feeding, health is far more important.
Good luck with your decision, its not going to be easy xx