We did a 17 night trip starting in Venice for two nights, then driving through Croatia visiting Plitvice, Dubrovnik, Mostar (Bosnia- Herzegovina) Montenegro and back up through Slovenia. It was amazing. If you can do a driving trip instead of a cruise with a port stop, you will enjoy it more. Less crowds, more to see. Driving there is super simple. And Slovenia is just so amazing!
The owner of the apartment where we stayed in Dubrovnik said she didn’t feel anyone ever really got a good glimpse of Dubrovnik by cruise because of the numbers of people that come in to a very small area. We enjoyed Dubrovnik a lot though.
I haven’t been to Greece but I believe from what friends told me that it is more cruise friendly.
Tough decision either way.