Give the 3rd day to Nancy or Colmar?

I visited both in 2017, without a car. I liked both. Nancy is quite a bit larger, and the Art Nouveau/Art Deco buildings (one of my major interests in visiting) are somewhat scattered. I planned no side-trips outside the city, which seems to be your situation as well. I needed more than one full day in Nancy, but I am sort of obsessive about tracking down early-20th century architecture, and I spent a good deal of time in two of the local museums (Museum of the School of Nancy and Museum of Fine Arts).

Colmar is smaller but there are all those little villages. (On my most recent trip I opted to take the train to Selestat, which is larger, less cutesy, but still attractive and less touristy.) Be sure to allow time to see the Unterlinden Museum in Colmar, which has more to see than just the magnificent Isenheim Altarpiece. Note that Colmar is very touristy. I still liked it a lot, but more than one poster on the forum has been bothered by the crowds. It probably helped that I spent a good bit of time on the side streets, where there was often no other tourist in evidence.

I suspect you will be able to get by with two nights in Nancy, especially if you can finagle an extra half day there either on the day you arrive or on the day you move on to Colmar.

I see that the travel time by train from Nancy to Colmar can be as little as 2 hours or as much as 3-1/2 hours, by the way.

Acraven, the only side trip I was considering from Nancy was possibly Metz, but I want to be sensitive to my hubby’s jet lag, so that would be on the third day if we gave the extra day to Nancy.

I haven't gotten to Metz, but it's on my (mental) list for my next trip to that part of France. There are just way too many wonderful destinations in France.

We were in Colmar last year in Sept. We absolutely loved it. yes, there are a lot of tourists but it did not bother us. We spent 6 nights in a little apartment in city center. Even though there were a lot of people there it was peaceful in evenings and mornings. and since we had a car we went over the border to Germany as well.
One of my favorite days was spent riding an electronic assist bike on the Route du Vin. The little villages are like a fairy tale. We rented the bikes in Colmar, magical day!
Karen

I actually spent 10 days in Nancy. In those 10 days we took one day to Strasbourg by train, one day to Verdun by car, and one day by car over the Vosges mountains to Riquewihr and Ribeauville. So 7 were in Nancy itself. I did not get tired of Nancy, but we are serious walkers and enjoyed the city. It reminds me of a petite, calm Paris. We also enjoyed their little zoo, besides the obvious art nouveau architecture and the Ecole de Nancy Museum. We were there at the end of August in 2007 so things change, but there were not many tourists and we only saw one American that we noticed...because he was loud. I would put the extra night in Nancy, but I am a slow traveler. I like at least 3 or 4 days in a place, and usually much more, before considering day trips. To me, one day in Nancy would be unfortunate and hardly worth the effort. That being said, I've never been to Colmar so this is a one sided discussion on my part.

I think it really depends on what your interests are . Nancy as others have mentioned has a lot of art noveau sights, and is bigger. Colmar has that real middle-ages, half-timbered appeal and being in Alsace has a bit of a more German feel.

If seeing Alsatian villages (which are wonderful) is a priority the extra night in Colmar might be helpful. But the sights in Colmar can easily be seen in a long day alone, whereas Nancy is quite a bit larger. It's a real toss-up!

I usually prefer to travel by public transit but if I was trying to visit villages near Colmar I'd strongly consider renting a car, even for a day or two.