Wow, Isabel. I so wish that my amom had done what yours did. The family tree project was so depressing for me. I got a lot of info from my grandparents about my aparents families. My aparents were fascinated, since a lot of the information was stuff they didn't know about their own families. I ended up feeling like I had done an ancestry research for complete strangers...I felt no connection whatsoever to any of it.
Anyway, to answer your question, assuming the adoption is open, and if I had to choose, I would say the original family.
The problem I see with giving the child a choice is that, being an adoptee, I bet dollars to donuts that the kid would choose to do the adoptive family tree just to make the aparents happy. I know I would have.
I think it is much more authentic to do the family of origin. I know the concern is that then the child will feel like the aparents don't really consider him/her part of the family...but the kid probably already feels that way anyway. Why not give them the permission to take an interest in their actual ancestry.
ETA: I remember when I actually found out some interesting info on my family of origin after reuniting with my first mom, I went to my aparents and told them about it...they couldn't have cared less. That really hurt, because it felt like they didn't care who I was.
Just a suggestion to aparents...take an interest in your child's first family. It's who they are and (if they're anything like me) they want you to accept them totally, even their origins.