Santorini has more tours, sightseeing, and activities: excavation sites to explore, day trips to traditional villages around the island, winery tours, and boat tours around the volcano. Mykonos has some fine scenery, beautiful walks, and picturesque windmills – but in general, Santorini has more to see and do.
Is it better to go to Santorini or Mykonos for nightlife?
Nightlife-wise, Santorini is far more chilled out than Mykonos, but that's not to say you'll be bored when the sun goes down. Fira, in particular, has a vibrant nightlife and a few nightclubs to check out too. You'll also find all different types of food to enjoy, including authentic Greek cuisine and fresh seafood.
Should I go to both Mykonos and Santorini?
Santorini, if there is any way you can visit both, I would highly recommend doing that. These two islands are similar yet unique, each offering a slightly different vibe and experience. You could fit both islands on the shortest of itineraries. The ferry between Mykonos and Santorini is just two to three hours.