Personally I'd vote for Koeln.
Frankfurt has some nice spots, Altstadt etc. but not really better than Koeln. And Koeln has the Dom, Koelsch and the Rhine. There's usually some kind of event on down by the Rhine in summer, too.
Of course, if it's raining, you may prefer Frankfurt's set of museums... depends what you like...
So I either stay overnight in Frankfurt and spend a few hours in Cologne before getting the overnight to Paris or have a few hours in Frankfurt and then go to Cologne and stop overnight. Or only visit Cologne.
Booking the trains this evening! Ad astra per aspera
