Hm, working in Düsseldorf having relatives in Cologne and friends in Bonn - I tend to say Cologne over Düsseldorf over Bonn ...

But it's not only a question of weather. Okay, in sunny weather Bonn might be the nicer town, but it's much smaller.

It has been our capital for some year, but this not mean it has any flair of the big wide world, it's more a viallage - but this might be exactly the point why many people prefer Bonn over Düsseldorf. The people there are friendler and not that snobish than the inhabitants of Düsseldorf are sometimes - go for Bonn!