I'd actually focus more on the itinerary than the company, because a lot of the boats are pretty similar.
A few things I'd think ask to narrow it down:
If you want the "greatest hits" of Croatia, make sure you hit Split, Hvar, Korcula, Mljet, and Dubrovnik.
If you're trying to avoid some of the crowds, look for itineraries that spend time in places like Vis and Mljet.
If food and wine are your thing, I'd want some time around Korula and the Peljeac peninsula. Great wine, oysters, and seafood.
If you like nightlife and being out late, I'd definitely look for an overnight in Hvar. Totally different vibe once the day visitors leave.
If your idea of a perfect day is swimming in coves and relaxing with a drink, pay attention to how many swim stops they build in.
As for companies, I'd start with Sail Croatia or Katarina Line and then work backwards from the itineraries that appeal to you most.
If you're open to something a little bigger (still small by cruise standards), I'd also take a look at Windstar and SeaDream. They carry more people, but they have a really nice laid-back feel and can still get into a lot of the smaller ports.
If you're looking for 5 star, then Four seasons has 97 rooms and Aman has about half of that, and I believe they will both be in the Adriatic next summer.
Always happy to answer questions or help with the trip if you're looking for it! Feel free to reach out.
Luxury & Adventure Family Travel Agent
Personal Travel Concierge
Europe • Africa • S. America
www.voce-travel.com
Kyle.regitz@fora.travel