Santorini or Nafplio

If Santorini is a must see for you then by all means visit. If you are neutral between Santorini and Nafplio I would choose to go to Nafplio. Its a delightful mid sized city and you have the possibility for taking in one or two nearby ancient sights.

Some images taken last year.
https://www.flickr.com/photos/stanbr54/albums/72177720319228274/

I agree with stanbr who sums it up well.

Santorini is a spectacular and unique view, and I am glad to have seen it, but Nafplio is a much more interesting place to be and is also beautiful in its own right. With five days I would definitely opt for Nafplio unless you really really want to see Santorini.

Also, there are likely to be many fewer tourists in Nafplio than in Santorini.

Thanks all for the recommendations!