Hello there,
I can say nothing about Duesseldorf as I have only been to the airport there. But I have heard many things about it, which confirms to me that Cologne is the right place for people like us. For us, Cologne is just a wonderful city. Friendly, international and lively. Duesseldorf looks much fancier, but we love the warm-hearted grittiness of Cologne. But I have never been to Duesseldorf, nor know nothing about the schools there.
However, we do have our daughter at the International Peace School here in Cologne and we can wholeheartedly recommend it!!! We are so pleased that we have this wonderful option in Cologne. The school is in its first year, so is still in a temporary structure, and getting itself organized, but this makes absolutely no difference to the quality learning experience of the children. All of the parents I have contact with are equally happy, and it seems to me even the ones I don't have as much contact with (in the other class) are equally enthusiastic So, take a look:
http://www.if-koeln.de/enGood luck in your decision, and please feel free to personal message me for more info!