Greece… The land of ancient mysticism, the origins of the Gods (Zeus, Hades, and Poseidon), the place of men’s greatest buildings, and the country with one of the most breathtaking ocean views. Santorini and Mykonos are great places to visit, and you're torn between the two. No doubt, of course, both islands are incredibly beautiful with the white buildings and both are suitable for honeymoon trips. However, there are many differences.
In Mykonos, the beach is always nearby while Santorini lies above the water line, so it takes more effort to get to the sand. Mykonos has better beaches, so the best hotels are closer to the sea. Meanwhile, Santorini hotels and restaurants offer amazing view of the ocean from a higher altitude.
Santorini has more to see such as caldera and volcano besides the ocean. You can visit wineries, take day trips to a nearby city, swim, or dine in fine restaurants. There are not so many activities in Mykonos during the day except for going to the beach and try out the food. However, you can find lots of clubs, bars, and beach parties at night.
Both islands are family friendly, but not many attractions for children. Santorini has some activities the family can do together during the day.
Basically, both islands are amazing and it is possible to visit both with a ferry ride. It just depends on what you are looking for. If you are into scenery, sunsets, romance, and fine dining, go to Santorini – perfect for couples. If you want to party and go to the beach, then Mykonos is better. If you want it all, then go to both!