If you want to go to Santorini, then go there. Don't forget that someone may take a hundred or more photos of the same scene before they publish one, as I know from personal experience! Getting the light right plus no people in shot etc. The weather maybe mixed in early May.
Reality check - Santorini is hideously busy and suffers from far too many cruise ships disturbing what was once a peaceful island. There will be more Americans than locals there! I just did a random search for this May on two random dates and there were 10,978 and 15,088 cruise passengers expected in Santorini! That's far too many for such a small island. Santorini has been ruined, sadly.
Look at the Pelion or Kalamata, Paxos or Alonysos if you want to see the more laid back, more traditional Greece. March and April, as maylm suggested is not a great idea - you could get 2 weeks of cold, wet weather and if the seas cut up, the ferries may be cancelled.
Food is better in Croatia and the local wines too. Both are good value, except for Santorini and Mykanos, which are about double the price.
With Croatia, there is a temptation to dash around trying to cover lots of ground. However, with two weeks, you would have time for a week on Vis, possibly hiring scooters to explore plus on the mainland, exploring Split, Trogir and Dubrovnik in the other week. Plitvice is a long day trip from Split and best with an overnight stay.
Although I have had many good holidays in Greece, if you want quiet and relaxation, I would head to Croatia. The sea will be warmer in early June (or even warmer in September).