Istria is a peninsula with a lot of towns and cities. Zadar is one little city. They don't compare directly.
Why WOULDN'T you go to Istria? Are you thinking you might prefer Zadar for some reason? I didn't. I liked Zadar to a point but I greatly preferred Rovinj. In my case, I was driving south anyway, and Zadar was a stop on the way. It was well worth the stop, but I wouldn't drive all the way there from Slovenia just to see the town for a few days then drive back. (There are things to see nearby such as Krka National Park though.) Istria has towns like Rovinj, Porec etc. on the coastline plus lots of cute little hill towns inland.
As for Pula: I did a day trip there from Rovinj. Pula was OK but not exactly exciting. It's a big industrial port town with some Roman ruins in the center and a nice pedestrian area in the center with some charm here and there. But I had little desire to spend much time there. I'd seen better Roman ruins...in Rome.
In July, I'm sure all of these places will be busy with tourists. Rovinj will probably be very busy and lodgings expensive. Pula maybe will be cheaper as it's a big city, I don't know. That's the only reason I would stay in Pula instead of Rovinj, personally.
If it isn't on your radar, Plitvice Lakes National Park in Croatia is an amazing park, one of the highlights of the region, perhaps the highlight. It's worth a detour between Istria and Slovenia.