Both are amazing for different reasons most of which are online. Mykonos for the beautiful Chora (main town) and the beaches. Santorini for the views (not the beaches!).
If you miss out on one or both of these islands, obviously you will have time for something else to fill in your itinerary.
During normal non-pandemic times, both islands are very expensive and very crowded. If you don’t care so much about the first issue, the latter will bother you! Santorini is not famous for its beaches. Party Island is synonymous with Mykonos and Ios. Touristy, non-authentic, you interact with Athenian businessmen most of the time. In Santorini, most accommodations will offer amazing views. The famous Oia sunset? The question is how you get there in time to see it behind all these people with the canon-like cameras! These are all just facts for the first-time visitor to consider. We would suggest a visit to Santorini for two nights (1 full day) and then … just go! Maybe not Mykonos to save time for another more “Greek” island?