Opatija or Rijecka?

What is the rest of your itinerary? Neither are the best that area has to offer.

We're flying into Split and spending 2 nights, then a night in the Plitvica lakes area, driving to Motovun, Porec and staying 2 nights in either Rovinj or Pula then a night in either Opatija or Rijecka before continuing to Zagreb for our last night.

I stayed in Lovran, because I needed an overnight in the area but wasn't particularly interested in paying a premium to stay over in Opatija. Rijeka is a bigger city - didn't want to deal with the complexity on such a short visit. I picked up my car in Rijeka and walked through a tiny bit of it; wouldn't mind exploring it sometime, but it didn't have an obvious charm. I did spend an hour exploring Opatija's waterfront, which is really nice, but otherwise the town didn't call out to me to spend more time there. It's kind of ritzy, not my style.

Lovran is a little fishing village that didn't do much for me either, but it was convenient, and from there I was able walk along the Lungomare, a lovely seaside promenade that goes all the way up to Opatija and Rijeka. I enjoyed the walk - recommend you do at least part of it if you have time.

Opatija would be my choice. My husband is from Rijeka so I have spent many months there. It really isn't a tourist destination, more of a jumping off point. Opatija reminds me of Atlantic City but with a walkway along the water in lieu of a boardwalk. You can sit and have dinner/drinks at one of the hotels' terraces and look out onto the water. The other end of the strip is where all the clubs are located. They also have an open air theatre which shows movies in English. They also have a beach which they paved over a few years ago so no rocks. One night is more then sufficient.

Rovinj is about a three hour drive to Zagreb; Opatija is about two. Since you don't really need to break up that length of drive, I would add the extra night to Split or Rovinj to give yourself more time to enjoy them.

I'd add the night to Zagreb.

Add a night in Istria. Don't stay in Pula either as it is the largest and least attractive of the Austrian coastal towns. We always rent a villa in a central Istrian village. As Zagreb isn't far from anywhere in Istria, staying in Opatija or Rijeka is wholly unnecessary (especially as they aren't very special).