We have never been to Barcelona. We have been twice to Paris. Were would you spend a fourth night if you had to choose?
I don't understand the question. Neither place is worth just one night. Where are you spending the first three nights?
I wouldn't spend one night anywhere unless it was simply a transit stop.. half the day spent getting there , getting to hotel , checking in etc.. and then leaving in morning.. not worth it .Both Paris and Barcelona are huge cities.. and there is so much to do in Paris I have been hundreds of days over the decades and still not seen many things.. so you are certainly not "done" with it.. but the question is a bit vague. If you are already in Paris then stay in Paris.. if you are somewhere else.. stay there.. four nights is not exactly a long time to be anywhere worth visiting..
Are you indicating that you will be spending three nights each in Barcelona and Paris, and have another day to plan and want to know which city to add it to? Then I would choose Paris.
I'm also not clear on the question. Are you visiting both cities and wondering which one to spend an additional night in?
Sorry, I was trying to ask a question before a storm came up. We are looking at 3 or 4 days in Paris or 3 or 4 days in Barcelona. We cannot stay 4 days in each city.
Since you've already been to Paris, you might as well go ahead and get Barcelona out of your system. If you don't, you'll always be wondering about it.
I'm sure you won't run out of things to do on your first visit to Barcelona. In the intro to Rick's Barcelona book, he describes sightseeing options for four pretty full days in the city. (The suggestions for day four are only covered in that book, not the larger Spain book.) Or day four could be for a daytrip outside the city.
So you'll be in once city for 3 days and the other for 4 days. Got it. Stay longer in Barcelona since you've been to Paris several times. You can easily fill the time and you'll find it has a lot in common with Paris.
Noticed that you are from Denver. There is a travel group meeting in Littleton this coming Saturday at 10, several people in the group have visited those 2 cities. Check general Europe, Denver meeting for more details if you are interested.
Oops, just saw is not the Denver in Colorado.
If you end up going to Barcelona, try to be there Sept 19-24 for the La Merce festival. It's the largest of the year in Barcelona. Parades and music constantly on La Rambla for 5 days. We were fortunate to be there a couple of years ago during the festival. Very memorable.