Northern Italy Vs Crotia

I would pick Croatia because it would mix it up a bit since you’re already going to the Amalfi Coast, Rome and Naples.
A great one-week itinerary for Croatia starts in Split where Roman Emperor Diocletian’s Palace is located. After Split, take a ferry or catamaran to Hvar Town ( 1-hr) then Korcula Town ( 1.5 hrs) then to Dubrovnik ( 2 hrs). Mljet National Park and Lokrum National Park can both be reached by ferry from Dubrovnik. Fly to Rome from Dubrovnik for your return home. Getting around by ferries and catamarans will be a relaxing contrast to travel in Italy by trains. And Cinque Terre/Venice are overtouristed and always crazy crowded—even during September.
From Athens, there are some direct flights into Split, Croatia ( SPU) taking less than two hours.
You can check out flights at www.Skyscanner.com
You can get to Croatia from Italy by ferry as well.
Ancona, Italy to Split; Bari, Italy to Dubrovnik.
Just reverse the itinerary above depending on where you start. From Naples, Bari to Dubrovnik (. 7 hours) works best
www.CroatiaFerries.com
www.TheTrainline.com has train schedules and fares for Naples or Salerno to Bari, Italy.
Have a great trip!

Rome, Sorrento, Athens, Santorini, Naxos, Mykonos; if that's your taste and style there is very, very little in Croatia that compares. Northern Italy besides Venice there is quite a bit in the same style.

For Croatia you would be taking a totally different kind of tour. That could be good to, if you will enjoy it. I think the time of year might do a lot to make the final decision. Croatia best in May, June, September.

How do you plan to travel from Athens to Northern Italy or Croatia? Will you fly? I assume so.

Perhaps consider flying from Athens to Split or Dubrovnik and then Split or Dubrovnik to Rome. Ferries and other modes of transport will take forever.

What is a must see in Croatia? I think that depends on taste and interests, but I would consider Split, one of the Dalmatian Islands (Hvar, Brac or Korcula) and Dubrovnik pretty important. That said, I prefer smaller seaside towns like Trogir and Sibenik. There is also the Plitvice Lakes.

What is better between these two is a personal choice.

Another option is to take a cruise from Venice that stops in Split and Dubrovnik, then to Greece and the Greek Islands.

geovagriffith and Emily have excellent ideas, but depends on the time of year.