If you have been to all or most of these places but had to choose between "Itinerary A" or "Itinerary B" which would you choose and why? We have 2.5 to 3 weeks max. We can pick any time of year. Thinking late spring, 2022. We've never been to eastern Europe (or these parts of Germany) in our early 70s:

Itinerary A: Germany: Berlin, Wittenberg, Dresden, Elbe River Valley, Czech Republic: Prague, Cesky Krumlov, Slavonice, Slovakia: Bratislava, Hungary: Budapest

Itinerary B: Croatia: Dubrovnik, Karanac, Zagreb, Plitvice Lakes, Opatija Montenegro: Kotor Bosnia & Herzegovina: Mostar, Sarajevo Slovenia: Ljubljana, Lake Bled

Itinerary B with Dubrovnik, simply because I'd like to go there myself and I've been to many places already in itinerary A.

You're asking what other people would like but this trip is about you. I advise researching each itinerary a bit more and deciding what appeals to you the most. It's a bit like apples and oranges.

Both good list of sites and I've been to them all. The cities in A are bigger with wealthier museums, the cities in B are smaller and more walkable. Walking is very much normal in Europe. Late spring. May? I think would be nicer in B.

really none of those countires are in Eastern Europe anyway -all Central Europe

Itinerary A requires more travel time. I recommend all places on both itineraries, but pick destinations that are close together, so you don’t waste too much time on trains between them. 2.5 - 3 weeks is not long; you don’t want to use up your time sitting on trains. Assume you will come back another year to see the rest.
Itinerary B lists places not too far from each other. I would either choose that, or re-work A to keep the travel time to a minimum.

It depends on the type of travel you want. Itinerary A is more crowded because there is more to see in the large cities, itinerary B is more leisurely with a greater amount of time spent just walking around.

Itinerary B appeals to me more, seems to flow better.

In terms of transport, rail infrastructure is much better along the destinations of itinerary A. You can take coach of course. For itinerary B, you'll rely on coaches unless you drive. Slovenian railway is more well-developed than the rest of the places in itinerary B.

Destinations of itinerary A are also more urbanised than B.

Do you have themes in mind?

You do realize the OP hasn't bothered to come back/respond in 6+ months . . .