If I was ever blessed to be in the situation that I had Frozen embs that I would not use, I would definatly donate. I personally would not want anyone else to go though what I am going though. I also would rather not have to choose the couple either. Just knowing that they deeply desire a baby would be enough for me.
I also think it is easy for me to say that, as out of 4 IVF cycles I have only had one that ended up with frozen embies. So it is very unlikely I will be in that situation.

It is very interesting though as there are just so many factors to think about!