Both are excellent ideas, the open question here is "cuisse ou poitrine?". Apart from your preference for island vs. [close to] mountain, the main argument in favour or against one or the other is that you can get to Meteora easily by renting a car, you can go there early in the morning and, if you want to go back to your hotel, you can be back in Athens in the evening. Sort of too much for Santorini. But believe me, whichever you choose, it's going to be worth it!