You may choose only one...

Santorini is good for two nights, max. The beaches are not as great as Mykonos and it is very crowded, as is Mykonos. Plus, both are pretty expensive. Are you going because it seems they are the "it" islands? Have you thought of Naxos? It's big enough that you could easily fill 3-4 days and be wishing for more. Plus it's an easy flight/ferry from Athens. Bonuses-cruise ships don't port in Naxos so it's not busy at all and the beaches are supposedly the best in Greece.

We took the Heart of Greece Tour in September of 2017 and spent 4 nights on Santorini and 6 nights on Naxos. By far Naxos was our favorite and we didn’t want to leave! If we were to do it over we would have spent 2-3 days max on Santorini. For the true Greek experience Naxos has a lot to offer. They have a wonderful bus system to explore off the beaten track places. One of our favorite places to visit was the marble village of Apeiranthos. The cuisine on Naxos was the best we had in Greece. Visiting islands following the tour was a great way to relax before going home.

Neither. You picked the two most "party" islands, popular with drunk 20 year olds. Pick any other island.

Yes, you picked the two most heavily touristed and expensive of the Greek Islands.

Lots of other options that are just as lovely and a lot less expensive.

However, if you have your heart set on either of those two I would pick Santorini, even though I haven't been there.

When I had the same choice I picked Santorini for 3 nights. It was great!

Naxos or Paros would be better. Forget Santorini.

Thanks all. I did say we have never been to Greece... It sounds like Naxos would be a better choice and that is exactly why I asked this group!