Would you spend the day in Kalmar Sweden or go to Frederiksborg Castle and Kronborg Castle?
I don't have time to do both.
It depends on your itinerary. Is Kalmar on your overall route?
I would go from Stockholm in the evening, spend the night in Kalmar then go to Copenhagen in the evening.
Is it worth the stop for the castle and Cathedral? If I don't stop in Kalmar I would go straight to Copenhagen and have an extra day to go outside the city.
I have been to Kalmar Castle, not the others.
I am NOT a castle person, however, I really enjoyed my guided tour of Kalmar Castle. I didn't enjoy Kalmar itself that much. I would still strongly consider the Kalmar option though, because it is the more leisurely option. It nicely breaks up your trip from Stockholm to Copenhagen. If you aren't going to do that you might consider flying.
I assume you are driving. It’s a long, really dull drive. In my opinion, it would be better to fly. Get your castle fix at one of those in Denmark.
Well Kalmar is not really on route between Stockholm and Copenhagen by train. Stockholm to Kalmar is a 4bout hour 45 minutes by train with one change whereas the direct train from Stockholm to Copenhagen is 5 hours. Frederiksborg Castle and Helsingør with Kronborg Castle are easy day trips from Copenhagen.
I have never been to Kalmar so I won't be the judge as to what is best. However Kronborg Castle in Helsingør and Kalmar Castle are from the same historic period and architecturally similar. Helsingør also has a nice old town centre. Frederiksborg Castle is a bit different from the other two, since it more a palace than a defensive castle like the other two.