I think I would want my biological child back. Yes, I'd love and mourn the child I kept for 2 years. But, there are so many more issues to think about than the immediate present. Like biological issues. Medical history and complications. You have to address more than the present. Until you start having medical problems, you do not know how necessary it is to know the family history. Viability & options for transfusions, organ donations, etc.
You can't later down the road, knock on a stranger's door and ask for medical help, when essentially that entire blood line has been a stranger. There is a line in True Blood, "He may be kin, but he's not family."
The fact that one day, if you keep the other child, that child you are raising will have children, who are not biologically related to you. They will not, by blood, be your grandchildren. You don't know if your biological child would form any kind of attachment to you as they grow. Your real grandchildren would then be somewhere else as they grow.
I don't think you can bring in all the usual arguments and fundamentals about loving an adopted child as your own, and of course their children would be your grandkids, too, into this situation.
Adoptions are done with a lot of forethought and consent. In THIS particular instance, the consent was originally taken away when the babies were switched. You didn't have a choice about raising a baby that wasn't yours. You DID conceive of one that was, and all the dreams that go with that.
PLUS, that child you did conceive is very really there. How do you address that in the future? What if that family has more children? What if throughout your biological child's life, they feel they never really fit with that family? And they never really formed a bond with you & your spouse?
Then, there's always the issue, most adopted children usually want to know who their real biological parents are and are like? That there is always a hole in them - no matter how much they love their adoptive parents.
Could you really let another person raise a child you know is yours? A lot of this theoretical situation of leaving your biological child with the other family is based on the belief that you child would be raised well in that other home. What if they other family is financially far below yours? What if you realize the other mother is like Octomom? What happens when that child gets into trouble or does drugs in their teen years? What if there is later found to be child abuse & neglect? Or sexual abuse? The perpetrator claims it wasn't incest, as the child was not biologically theirs. You weren't a major influence in their life growing up, can you live with the fact your biological child got messed up, and had you been a part of their life, that may not have happened?