We want to visit at least 2 islands including Santorini. Are we better offer using ferries or doing a 4 night cruise?
You won't be able to see much on a 4-night cruise. And part of the experience is just sitting in a cafe somewhere for as long as you want enjoying the atmosphere, and strolling around while people-watching and window-shopping without having to watch the clock so you don't miss the departure of your cruise ship for the next stop. I'd much rather use the ferry system and plan my own itinerary so I could come and go at my own pace. Islands close to Santorini that are on the same ferry route include Paros and Naxos. From either of these you can also take a daytrip excursion to the World Heritage site of Delos Island that will include a stop in Mykonos Town for a couple of hours. And from Paros you can also take the local small shuttle ferry to Antiparos, which is a quaint and small island with a friendly village vibe.