From Slovenia-North Italy or Croatia?

Go south to Croatia since you'll already be in Slovenia.

You can always go to Italy.

I agree that you're more likely to fit Italy into a future trip than Croatia, so I would rent a car to poke around Slovenia and the northern half of Croatia (or to drive across the Italian border, if you wish). I had no trouble driving around the area, except when I made the mistake of trying to drive along the coast on a Saturday in August (peak traffic time).

That's also because public transport across the border to Italy takes longer than you might guess, unless you just book a private shuttle like GoOpti. Croatia's main train line goes down the center of the country, not along the coast, so buses usually figure into the public transport mix there. There are ferries to islands, but the long-distance car ferry no longer runs down the Croatian coastline.

The Plitvice Lakes National Park in northern Croatia is a magnificent destination if you're lucky with the weather. It is mobbed by daytrippers during peak season, when it's essential to spend the night before your visit very close to the park so you can get an early start. I don't know how essential that would be in March, but it certainly would not hurt. With an early start you will not need all day at the park.

At least in summer, there's decent public-bus service to the park from both Zagreb (nice city though not on the coast) and Split. I don't know how much service there is in March. In any case, you'd need to select your lodgings carefully for proximity to the park if you didn't have a car.

I concur with Plitvice National Park. We also enjoyed the Istrian Peninsula and the town of Motovun. It's a village atop a hill. It was once part of Italy, and birthplace of Mario Andretti. Truffles grow in the forest nearby. We enjoyed the truffle dishes that were prepared.

Public transportation: Italy.
Rental car: Croatia.

Thank you all so much for the advice! We are going to do Croatia and rent a car and will for sure be staying at least one night in Plitvice. Thanks again!

How about this: drive from Ljubljana to Istria (Croatia) and spend a few nights. Then drive over to Venice for a few more before driving back to LJU. I'm basically doing this same trip in July although we're headed to Switzerland from Venice.