Skansen

If you are culturally and historically interested I would visit both actually because you will see some commons and also some differences in the cultures. From my personal preferences I would spend the time for both deep dives because both cultures as well as the Danish have so much to explore.

In Germany we can still experience parts of both cultures because parts of Germany were in very earlier times Swedish and Danish territory. The relation to Norway more than 500 years ago you can still see today by visiting the rebuilds of Tyske Bryggen in Norway (UNESCO world culture heritage).

Full disclosure, I love skansens! I’ve seen both Stockholm Skansen and the Norwegian Folk Museum. Stockholm Skansen is one is the earliest open air museums and served as a model for many others around Europe. But I actually prefer the Oslo one, if you only have time for one, because it has a greater variety of time periods...and especially because of the wooden stave church. It is also a bit smaller so easier to cover if you’re pressed for time.

MarkK lays out the reasons why I never skip an outdoor Skansen museum...Lviv is my favorite.

I think you would really enjoy Skansen. We were there last winter and loved the various exhibits which included amazing folk music and great information on what life was like for people living in different regions of Sweden. I thought the animal exhibits were very well done also and it was a great day spent wandering around the grounds there. Enjoy your trip!

My husband and I have included both of these in our itineraries on our trip next month. It is our first trip to Scandinavia and each seems to be a 'don't miss' for us. We're looking forward to our experience in each of these outdoor museums.

My wife and I just returned last week from a similar trip, and visited both outdoor museums. We really enjoyed both, but if I had to choose one it would definitely be the Norwegian Folk Museum. It is more compact, and felt more authentic than Skansen, with buildings of the same era and location clustered together. Skansen is much larger, and you could spend a whole day wandering around, but a lot of space is given over to small farmyards, the zoo, and commercial areas. Both sites have regularly scheduled demonstrations, and very helpful attendants in period costume.

I like Barbara's "full disclosure" - and I share it. Never Never pass up a cultural anthropological experience.
I also prefer the Oslo to the Stockholm, but I'd include a revisit to either one if my travels took me there. Both are excellent.
There are "one trick pony" museums in many cities - excellent, fascinating, totally worthwhile and "must sees" - but, in my opinion, a "one and done" experience (i.e.: Vasa Museum in Stockholm or the Viking Ship Museum on the Bygdøy peninsula in Oslo). I cannot say that about any open air museum I've ever seen - the first being the Shelburne in Vermont, which I visited when I was fifteen.
High on the list of recommendations -
Szentendre Skanzen Village (just upriver from Budapest - on the Buda side) and the Village Museum in Bucharest and - if you get hooked on these adventures, just up the rails from Oslo, there is the excellent Maihaugen in Lillehammer
Maybe I could plan a European Open Air Museum tour for my self - I've never been to Ukraine, and Barbara's recommendation is noted.
Here's a link to Rick Steves' thoughts on the subject: https://www.ricksteves.com/watch-read-listen/read/articles/europe-open-air-folk-museums

We (50 something couple with a 17 year old son) were there recently (late July). Our opinion is Skansen is overrated and does not deserve the three triangles. It was constructed for the Swedes to see their historical way of living and to admire curious animals. We highly recommend Vasa Museum, the museums in the Royal Palace (armory, apartments, treasury), and the City Hall. The islands are also nice. Walking around Gamla Stan is an experience though very touristy.

We’ve been to both and enjoyed them. If you have time, do both. Skansen covers a wide range of time periods than the Oslo museum. The costumed interpreters at Skansen are very knowledgeable but you need to interact with them. The Oslo museum had a stave church while the one at Skansen did not. Both have performances.

Stockholm is my favorite European city (of the ones I have been to). But, I admit I was disappointed in Skansen. It may be because I live very close to an open air museum (Hale Farm and Village) and at Cedar Point in Sandusky there is a psuedo open air museum section. So, maybe it wasn't anything new? I was there an afternoon in June. Many of the interactive displays were closed. There was a glass blower working, but I have seen that a dozen times. Also, I looove zoos. I found the zoo depressing. There were a few reindeer that were boiling in the heat and they didn't look well. That ruined my whole day.

I did like the runestones that were there.