When we considered having a "known donor" we made our decision based on what we thought might be best for the child, not us. We decided that "known at 18" was the way to go and found a sperm bank that provided that option. As it turned out, we ended up doing an Open Adoption, and our daughter (now 9) knows the birth dad. He's visited us 3 times since she was born. We talk to him on the phone a couple times a year, and we email him about the same amount.
What's interesting to me is that our daughter really wanted to know who her birth dad is. (she also knows her birth mom, but since she's already got the mom thing covered, in many ways having a known birth dad has more juice behind it for her).
Some kids may really care to know, other kids might not care. But it's good to leave an option open, is my feeling. Good luck and do what you and your partner think is the right thing, with your child in heart and mind!