Piran or Rovinj

I think the answer depends on your plan while you're there. How many nights would you stay in either location? If you have a car for two days, does that mean just the one night in between? Where are you going next, and where do you have to drop the car?

In general, I think Rovinj is a better destination than Piran, and if you want to explore Istria, it's a smarter option because you'll be in Croatia already.

As noted above, it depends on where you are coming from and where you are going to.

There is a border between Slovenia and Croatia which can be time consuming to cross in a car.