We're not at thsi point yet, either, but it's something we've talked about.
At first, flush with joy at finally conceiving, we were keen to donate. But after DS came along we changed our minds. The idea of having 'our' kids out there - but not ours - suddenly became much more difficult to contemplate.
I prefer the idea of research or practice for the lab technicians than simply allowing them to succumb. DH is not so sure about that. We only have 2 and hope to go again - one day - so it may be that we won't have to decide any way.

It's a tough one - take your time and talk it out. I hope you can find the answer that sits best with you.