Both cities are beautiful. You can't go wrong with either. Given their close proximity, you can easily visit both based in either city.
I guess Cologne wins most tourists versus Düsseldorf simply because of its world famous Gothic cathedral.
Both are of equal interest. Personally I prefer D-Dorf for better beer, nightlife and a few mins closer from where I live.
Cologne wins hands down. Düsseldorf is IMHO highly overated, and I would not call it "beautiful". Unless you are into either heavy beer drinking, or very posh shopping and yuppie culture, there isn't much reason to visit Düsseldorf. The region has more interesting destinations to offer.
I´m somewhat biased too, but after some visits at Dusseldorf I have noticed that this city has some very nice fascets too.
So I think you won´t go wronge with both cities, and it´s just a short (train) ride between them.
I don't think either place is beautiful, being mainly post-1945 reconstruction but I'd vote for Cologne over Düsseldorf esp. with kids that age (various kid-friendly museums include Chocolate and Sport as well as the Römisch-Germanisches.)
Düsseldorf when we were there last autumn was suffering from major redevelopment work in the centre so not even the Kö was beautiful (and as blandine says, there's the question of how much posh shops appeal in any case) . I enjoyed visiting the Goethe Museum but that's not of huge appeal to many!.
In fact I most enjoyed the day-trip i made to Zons http://www.dormagen.de/432.html and to Neuss which are both a lot prettier http://www.neuss.de/tourismus I found out about Zons from a tourist brochure, where it was described as being rather like Rothenburg ob der Tauber (without the tourists!)Edited: 30 March 2013, 11:13
Thanks for the help!
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