To sum up: a lot depends on your own stamina because island hopping means packing and unpacking, getting on and off buses and ferries. Limit your choice of islands to perhaps one or two less than you think you can manage. Maximise transport links to avoid backtracking or port-transferring and since you are traveling high season be aware that you will usually need bookings ahead at most places. It is possible to turn up on an island and not find a place to stay or have to make do with a third-rate option.
It’s a tough call. The good news is that whatever you decide, it will feel like the right thing after you’ve done it. You’ll almost certainly say to yourself, “I’m so glad we saw both islands” or “I’m so glad we had 6 full days on Santorini.” In deciding I would ask whether a return trip to Greece is likely or possible in the next couple of years. If so, then spend your 6 days on Santorini and explore that island, then return to Crete at some time in the future for a deserving 7 to 10 days. But if this is a one-time deal for the foreseeable future then do 3 days on Santorini and 3 days in Crete. (Whatever you decide don’t do a day trip to the other island as it’s not worth the time and effort.)
Like Imperial Airways, Air France and KLM's early growth depended heavily on the needs to service links with far-flung colonial possessions (North Africa and Indochina for the French and the East Indies for the Dutch). France began an air mail service to Morocco in 1919 that was bought out in 1927, renamed Aéropostale, and injected with capital to become a major international carrier. In 1933, Aéropostale went bankrupt, was nationalized and merged into Air France.
Airlines have substantial fixed and operating costs to establish and maintain air services: labor, fuel, airplanes, engines, spares and parts, IT services and networks, airport equipment, airport handling services, booking commissions, advertising, catering, training, aviation insurance and other costs. Thus all but a small percentage of the income from ticket sales is paid out to a wide variety of external providers or internal cost centers.
Excellent choice of islands, though getting between them presents one or two challenges (they are not as conveniently connected like the Cycladic or Dodecanese islands so island hopping is not as popular in this island group). All three islands are served by airports with year-round connections to Athens and in the Summer with international charter flights from Europe. Let’s take the pros and cons of each island.