Paper, absolutely.
When I learned most of the kids in my daughter’s second grade class had no books at home, and three was the most, one kid having that many, I went home and sorted through her library of well over one hundred. She was over two years ahead with her reading, so she had plenty she didn’t read anymore.
I took the stack in the next day, and let each kid choose one, then another, until the stack was gone. Owning a book they could hold, and could call theirs had every face in the room beaming, except a girl who thought I mistreated her, giving away some books.
It means more, turning the pages, scrutinizing any pictures.
If they have ownership, putting their name in it, and having it to keep is like treasure claimed.
Definitely paper.