I prefer Düsseldorf's alt over Köln's Kölsch, but 19 out of 20 times? Bah. Kölsch is subtle, and there's a lot to be said for the the complexity that goes on there but is not obvious. I'd say it's a more rewarding beer in that regard. Alt is more obvious, and it's wonderful stuff. I do prefer it, but I like having both. I'd be splitting my time far more evenly, especially since they're half an hour apart. Besides, Cologne's a better city than D'dorf.
-Steve