Monemvasia

Both are extremely attractive visits, although they are not physically or historically comparable. Is there a difference in your transit mode between them? I ask because we did not rent a car for either, and wouldn't call either easy-to-get-to. Both sights are evocative vistas, one inland, the other coastal.

Monemvasia would probably take more time, if you elect to climb the cliff stairs to the "upper level". Monemvasia is the setting of at least one of the Aubrey-Maturin sea novels. And Delphi figures in a lot of mythology. We're glad we saw both, although not on the same trip. Interestingly, on a WIndstar cruise, the ship could not get into Mykonos because of brutal rainstorm (both of us seasick), and Monemvasia was the "substitute port." But there was nothing second-rate about it at all. Great visit.

Edit: To add, unless you read a lot of mythology and Mary Renault in your youth, and have read or seen a lot of classical drama, Delphi could look like another bunch of stones to some visitors. (Not slamming, just observing.) Monemvasia is much more spectacular.

You should know what you are most interested in.

Delphi is a religious sanctuary and an archaeological site that was built about 1000 years before the fortified city of Monemvasia which was built for military purposes during the Byzantine Empire, I don't see how one can be recommended over the other given that they have nothing in common except being located in Greece.

Whichever is easier to get to. Honestly.