This is a difficult question to answer as both Mykonos and Santorini are such beautiful places. They both offer stunning scenery, luxurious accommodation, and plenty of activities and attractions to enjoy.
If you're looking for a truly special holiday experience that's highly Instagramable, then Santorini is the perfect destination for you. However, if you're looking for a more relaxed holiday, then Mykonos might be a better choice.
It really depends on what you're looking for in a holiday. Both Mykonos and Santorini are wonderful places to visit and you're sure to have an amazing time whichever island you choose.