Choosing between the two is difficult as I like them both, but I would go with Josephina.
Last Christmas we gave Molly to our 6 year old who was wishing like crazy that Santa would bring her to our house. The joy on Christmas morning was a sight to behold.
She has been madly in love with Molly ever since and is rarely without her - except at school. Even though she is only 6 she has proven that it doesn't really matter the age if they are ready for it. Yes, Molly's hair is quite the sight, but I always say it is a sign of a well loved doll. I guess I really have to give my dh the credit as he is the one that pushed for it. (I too was hesitant due to age)
This year, she is hoping for Emily and has already sent 3 letters to Santa beginning mid-summer. Bless our postman for picking them up. Needless to say there will be an Emily under the tree.
These are wonderful dolls and hold up well. Also, if you ever get a chance to visit an American Girl Place it is well worth it. The historical area is very educational and there is so much for the girls to do. Maya asked to go there in lieu of a birthday party, and it was a day to remember.
Sorry for the long post, but you won't regret the purchase whether it is Josephina or Kaya.