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
The question is also, which of the two cities offers a better connection to Paris. I personally would go to Köln, maybe Koblenz and then to Frankfurt, as my guess is, you have there a better connection to Paris.
I seem to have figured out the itinery now. I'll arrive in Frankfurt early am, spend a few hours exploring after I dump my bag in a locker. Go to Cologne and stay overnight, spend a day exploring and go to Paris late aft, and get the overnight to Toulouse.
Arrive all worn out at afew's! Ad astra per aspera