Both are great ports, but for trips to somewhere else. I suppose you could just walk around town and get the atmosphere, but you are in one of the cradles of civilization and IMHO it is a shame to miss the chance to see the sites.
Katakalon is the port from which you can visit the Temple of Zeus at Olympia, site of the original Olympic games.
Nafplion gives you access to Mycenae and many other ancient sites.
I highly recommend Nick of Greek Taxi!
Check out our trip report and pictures at
"Black Sea Cruise 2009"
Have a GREAT cruise!