I'm going to forward your message to a parent to answer because she had the experience.
Children are so individual it is impossible to make a placement decision based on a diagnosis.
I do have a brother in law who has Down Syndrome and if he hadn't been included with children his own age he probably wouldn't be as motivated as he is today. He believes he can do it all and that keeps us all going believe me. I have also worked with very young children with disabilities who I also believe have been motivated and are learning because they are included with their same age peers.