Istria or Dalmatia ?

I'd pick Dalmatia.

Both are worthwhile and both would probably appeal to you all, but because of the gorgeous islands off the Dalmatian coast, with some of the clearest, bluest water I've ever seen (boats look like they are suspended in midair), Dubrovnik, Split, Trogir, and if you can squeeze in a side trip to Plitvice Lakes National Park or to Mostar, you will get all your boxes checked.

Though there may be a slight edge on historical sites with the larger expanse of Dalmatia, Istria does have islands close by--Brijuni in Istria, and Cres, Losinj, and Krk in neighboring Kvarner region.
I would base the decision on flight and time availability, as you can make a wonderful itinerary in either. Consider Istria would mean driving to hill towns (unless you stick to the coast) whereas Dalmatia can be explored well by ferry and bus.

both areas have their pluses. Istria opened a water park a few years back which could sway you. But, if I had to choose, I would pick Dalmatia. The ease of getting from place to place is better in Dalmatia. Also the airports and flights are better into and out of Split and Dubrovnik.

Ideal trip if you have a few weeks, you could do both areas quite easily, best fly into one airport and out of another. Look at all the flight deals, also include Venice as entry point.

thanks for those answers !

With 12-14 nights can we do both ? Venice to Dubrovnik to Venice...is it doable ?

Because you keep posting in separate threads under two aliases (are you bucephale as well?), folks may not be aware that your plans include Slovenia and the Dolomites. Istria makes a lot more (geographic) sense when your trip is also centered on Slovenia, Venice, and the Dolomites. Driving all the way down to Dubrovnik and back from Venice is stretching yourself way too thin (plus the backtracking). In the summer, you'll be sitting in major traffic while driving along the coast. Why waste time passing over Istria when it's more accessible? You can save Dalmatia for another trip and pair it with Montenegro or Bosnia that makes more sense than Slovenia and the Dolomites.

Alternatively, why not just see Croatia itself for your entire trip? It's got a very long coastline and several interesting regions. That way you won't have to deal with the rental car drop-off issues - the car will be used within a single country.

My advice is to lay out your trip on a day-to-day calendar and write down all the distances from Point A to Point B to Point C, etc. You'll realize you're trying to cover too much area, even with a generous one month vacation timeframe.