we plan to drive from split to istria where we can stay for 3-5 nights. we are not sure which town (or towns) to stay in. can anyone recommend opatija or rovinj.... or should we stay in both?

With 5 nights you could stay in both. 3 nights is good for Rovinj. Be sure to visit Pula and don't stay at one of the big hotels down the bay (ie Park, or other). It's much nicer to be in the old town.

Hi
When we were in Istria, we stayed in Porec and Rovinj. Porec was larger, very nice place to visit. It's an old Roman military camp with a basilica that's on the UNESCO list of world heritage sites. We stayed there and then traveled to Rovinj and Trogir.
We preferred Istria to the rest of Croatia. Istria has a more of an
Italian Italian influence.

thank you both. any recommendations on "old town" hotels? did you visit opatija?

I stayed at Hotel Adriatic and enjoyed it. It's not fancy and is a bit dated but the location is great and the breakfast (included) was very nice.

Hi
I'm sorry, but I can't remember the names of the hotels. In Rovinj, it was within walking distance of the town, we had 2 meals a day at the hotel and lunch in town. Very close to town, it had a swimming pool and other facilities.
In Poruc we stayed at a similar type of hotel but chose to only have bkfst.