Piran or Rovinj

We stayed in Piran this past June it was perfect for us-we used it as a base to go to the Caverns and the Lipenzaner Stables(don't miss!).
Note that non-resident cars are not allowed in the city except to load and unload, however there is a free buse from the parking garage(€17 for 24 hrs)to the central city. We found that the easist way from the city to the garage was a walk along the shore to the garage, and then take the elevator to our car rather than the hike up the hill through the entrance.
Finally, haven't seen the write-up in the WSJ about Piran while we were there but suggest you check it out-appears it is "still off the grid" for Americans.

Where will you be arriving from, and where are you headed next? That might affect recommendations.

If I have to choose, Rovinj by a little.