We will be spending 13 days in Croatia and Slovenia in late May and need to choose between time in Korcula and Hvar vs, time in Rovinj and the Karst region of Istria. Which would be preferable at that time of year? We will also be going to Dubrovnik, Mostar, Split, Plitovice, Ljubljana, and Lake Bled. Thank you.
It sounds as if you don't have much time left, in which case one island might be what I'd do. I think Istria is best if you have at least 3 full days there (4 nights).
For sure you want visit best spots. Korcula is island gem. I will rater go to Korcula, Marco Polo birthplace. Just google the photos of Korcula and you will know what I am talking about. Simple wonderful route: Plitvice Lakes (overnight), Rovinj, Split, Hvar Korcula and Dubrovnik. Slovenia if you are going to land in Ljubljana/Zagreb. Than rent car and go way to Plitvice/Rovinj. And leaving the car in Dubrovnik/Korcula.