There are too many variables to write an answer. Would the girls bond better? Would the moms feel tied down? Who could provide better economic and emotional support? what might be the relationship with siblings, classmates, etc. in each home?
I can only relate a story about my friend who adopted a daughter. Daughter always knew she was adopted, but had no contact. The birth grandmother wanted to see her granddaughter before she passed. Arrangements were made. Adopted child was angry, frustrated, sullen during trip to see grandmother. After the visit, the child thanked her adopted mother for giving her advantages she would never have had. birth parents have had no other contact. My friend also adopted a son. He located his birth parents because medical genetic information was needed. Through his contact he has found brothers and sisters who have become very close. Birth father denies all children.
Message--each case is different and at best only generalizations can be made.