I voted the Dad's. I know nothing about he legal ramifications, but I think the father's name is the right thing. That's the traditional thing (in this country, anyway). It doesn't matter the circumstances of our conception or if our parents stayed together. It's a link to the other half of our DNA. At some point, this child will want to know his father and his father's family. It doesn't matter if the father is a good guy or not. If he ever steps up to the plate. If he ever pays child support. This is his child. Grandad should butt out and let this woman start making decisions. She doesn't need to feel like she is betraying someone when she makes a choice. To me this doesn't even need to be an issue. Father - Dad - sperm donor -- whatever you want to call him is responsible for half the DNA of your grandchild.