Wow, really sympathise with you BMDAFL, very difficult situation.
I think I would probably want to know, but for practical reasons:
a) I would want to make sure that someone was there to look after my children
b) I would want to make sure my 'affairs' (i.e. finances, life insurance etc.) were in order and that I had made a will etc. so that I didn't leave DH and family with a big confusing mess
c) I think (but am not totally sure) I would want my family to be able to be aware and prepare for this emotionally, especially kids. It is never easy no matter how much one might be 'expecting' the death of a loved one, but a complete shock can raise a whole load of different issues to a 'semi'-planned/expected one.
d) I am unsure about this one, but I think it would help me to make sure I really do value the important stuff and make each day count (I know that's cheesey, sorry!) and make sure my family knew I loved them etc.
Having said that, you could take positive and pro-active steps to do most of the above things without actually finding out, so the choice is still a difficult one. I think for me, knowing would spur me into action on the practical stuff, as I am a terrible procrastinator.
I guess you need to think about how you might feel if you know you do have this condition, compared to not knowing. From the way you have described it, it sounds like there is a possibility of sudden/early death, but also a very significant possibility that this might never happen and you could live to be 93 and wrinkly, in which case, you will still not really 'know' IYSWIM. Think about this too, as if you find out, you might have two sets of anguish - the knowledge that you have it, and still being completely in the dark about what this might mean for you.
Sorry you're in this situation and hope you make a decision you are happy with.
D