Denmark or Norway

First of all let me say that as a Dane i am totally bias in this and in the end it is your call. However it seems you have done your homework and are well aware of the main draws of both countries and how they differ. Both countries have Cultural spots, viking history and churches, since they have a common history, though Norway definately has the upper hand when it comes to sceneic views. Regarding vikingships, the vikingships in Oslo are much more well preserved than the ones in Roskilde, but the viking museum in Roskilde is a much better museum. If you want to know more about viking shights in Denmark, i'll be happy to point you in the right direction. Also do you know where in Denmark youir wives great grandfather is from?
To put it simple, Norways biggest draw is the nature and the western fjords, Denmarks biggest draw is the cities and castles and palaces. To make the most of a tour of Norway you need a car and some more time than 6 days, if you only go there to see Oslo (the least interesting capital in Scandinavia) and Bergen and a short fjord trip, you will miss some of the bigger draws.
With 6 days in Copenhagen you could spend 3 days in the city and take day trips to Roskilde and see the vikingships, Hillerød for Frederiksborg Palace and Helsingør for Kronborg Castle (the one from Hamlet), all by train. In my mind these are some of the main highlights of Denmark.
But like i wrote in the beginning, this is a Danish biased point of view, so in the end it's your call :-)

I think there is much more to do in Denmark than Norway, particularly Copenhagen. There is plenty of culture, history, churches, etc. in Copenhagen.

The Copenhagen card covers many admissions. It also covers transportation around north Zealand, including trains to Roskilde (Viking Ship Museum and Cathedral), Hillerod (Fredericksborg Castle), and Helsingor (Kronborg Castle). We took a "private train" also included on the card (but private because it's on a different track) up to a beach in Hornbaek after visiting Kronborg.

Oslo is nice but much smaller and fewer sights. Norway may take the prize for scenery, but only in the fjords that are a long way from Oslo.

We spent five days in Copenhagen and I felt I could have productively used twice that. I felt one full day in Oslo was enough to see the sights.

FYI, a few more answers are on the Norway version of this thread: https://community.ricksteves.com/travel-forum/norway/norway-or-denmark.

As an American who visited both recently with young kids, here is my two cents. Both countries are wonderful choices to bring a young child. If you go to either country thinking that you will experience it through your child's eyes and will not check off many of your desires (but will return when you can to do these things), then you will create a unique and special memory with your family.

There is really no right or wrong way, but since you rank Norway higher, according to your comments, I would probably go with Denmark. It will likely be easier to go at your child's pace and experience the culture as a family if you aren't thinking about checking off life goals at the same time. Go to the parks in the residential areas and let your child interact with local children. Go to touristy but lovely Tivoli and let your child hug the bear and go on the delightful playground.

I see you posted this in May, so perhaps this advice is a bit late. Would love to read about your Danish or Norwegian adventure!