Rouen or Reims?

What time of year, will you have a car, how long will you be there and what do you want to do while you're there? I've been to both and like them both, but for different reasons.

It will be June. We will have a car. In the case of Rouen, we would be on the way to Le Mans from Giverny. In the case of Reims, it would be connected with a trip to Verdun. The itinerary I'm trying to develop can accomodate 2 nights and one day in either place, but not both.

Lo, I don't think either is worthy of two days. I would evaluate on the grounds of the surrounding options. Each option has a good, close place to see in addition to the primary destinations. For Reims, the other place to see is Laon. For Rouen it is Honfleur. I do prefer Rouen to Reims and Honfleur to Laon. Then you have Giverny and Le Mans vs. Verdun.

Logistically it is easy to visit either as a day trip from Paris by train. So I think you can see both, if you like, by spending one rather than two days in each.

While Rouen is the more charming of the places, I would pick Reims for the two days. In Reims, you can see the Museum of the Surrender, tour champagne houses and see the cathedral. Actually, since you'll have a car, I 'd recommend the champagne houses in Epernay instead.