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.)
Growth rates are not consistent in all regions, but countries with a de-regulated airline industry have more competition and greater pricing freedom. This results in lower fares and sometimes dramatic spurts in traffic growth. The U.S., Australia, Canada, Japan, Brazil, India and other markets exhibit this trend. The industry has been observed to be cyclical in its financial performance. Four or five years of poor earnings precede five or six years of improvement. But profitability even in the good years is generally low, in the range of 2–3% net profit after interest and tax. In times of profit, airlines lease new generations of airplanes and upgrade services in response to higher demand. Since 1980, the industry has not earned back the cost of capital during the best of times. Conversely, in bad times losses can be dramatically worse. Warren Buffett in 1999 said "the money that had been made since the dawn of aviation by all of this country's airline companies was zero. Absolutely zero."
Germany's Deutsche Luft Hansa was created in 1926 by merger of two airlines, one of them Junkers Luftverkehr. Luft Hansa, due to the Junkers heritage and unlike most other airlines at the time, became a major investor in airlines outside of Europe, providing capital to Varig and Avianca. German airliners built by Junkers, Dornier, and Fokker were among the most advanced in the world at the time.
Since St. John is part of the U.S. Virgin Islands, U.S. citizens can visit without a passport. There are quite a few bargains on both St. Thomas and St. John right now, so it’s a good choice for travelers on a budget. Snorkelers should not miss visiting Trunk Bay, one of the best snorkeling spots in all of the Caribbean for seeing amazing marine life.