I agree, that is a very tough question and so much to have to consider. This is not just a casual "What If?"..this is a life changing "What If?".
I know that there are so many advances in the treatment of cancer, and I know many people (including myself) who have survived many different forms of cancer (including breast cancer). But, to have to undergo so much in order to prevent something that may never happen; that is a very scary prospect and a decision that cannot be made in a matter of days. There is so much possible suffering on both ends..the disease and/or it's treatments.
At this point, I really cannot say what I would do. However, I do know that there is no way that I could make the decision by myself and I would have to consider what I want out of the remaining years of my life. There is sooooooo much to be considered. Take your time, talk it over with your loved-ones, consider every possibility and please know that we are here for you, whatever you decide. I wish you the very best!
Peace and Blessings
Look For The Good and Praise It!