We did a two+ week trip to Greece and Turkey 3 years ago (right in the midst of Grexit discussion). Briefly:
3 days in Crete (car rental)
Ferry to 2 days in Santorini (car rental)
Fly to 3 days Athens
Flew to 1 day to Izmir (really just to see Efes so car rental)
Flew to 3 days in Capadoccia and Kayseri (car rental)
and 3 days in Istanbul.
We generally flew either 6.am or 9.pm and all flights <1 hr.
General thoughts, Turkey was far more enjoyable and interesting and memorable, but I don’t think I need to go back in the same way that I need to get back to Greece. Istanbul, Athens, and Naples (maybe you’ve been?) are similar in that they are major cities first and then tourist destinations second and are all on the grittier side than much of Europe that is more familiar. Turkish food is truly amazing- always request whatever the server recommends or the house specialty. Several times we had to wait an extra 15 minutes while the chef whipped up a favorite recipe.
English is 0 problems anywhere- maybe in Izmir but not really.
Capadoccia (and its many many spellings) is probably the coolest, most interesting place but I like to do hiking and walking about and the underground houses & cities were amazing!
Efes is not to be missed- our biggest regret is not having done some of the other destinations nearby- like Bodrum and the limestone pools. 3 weeks would have been better.
Cash (and U.S. $) in both countries commanded a huge discount- particularly when negotiating, but we were there when both countries were hurting financailly and there were ATM limits for citizens, but a good meal for 4 in Istanbul was ~$10-15.
Istanbul and Athens have much to see and do and all of it is at least somewhat redeeming. But pick and choose what you want to see and eat ahead of time- choose a hotel close to the city center(s) as walking about at night is what makes a city come alive!
But I think 3 days is about right for either Istanbul and Athens- both could really be done in 2 days if needed but rather than spend a 4th day, go elsewhere!
If you can tolerate the Grand Bazaar for more than 30 minutes, I am not your best tour guide- I liked the 60 historical artistic copies of the Koran at the museum on the hippodrome and the local tour guides mosque descriptions far more!