Is there a big difference in weather April vs. May in Turkey and Greece?
I don't know about typical weather but I just returned from both - I spent Easter, Apr. 8 in Istanbul where the weather was beautiful. It was warm days, no jacket needed, to cool nights, light jacket felt good. I was Athens, and Volos, the next week and it was about the same. All of the parks were green and the trees and shrubs were in bloom. Istanbul was having a tulip festival so I suppose they anticipated nice weather.
May is much warmer than April, and this can make a difference depending on where you're going and what you're doing. If you want to swim on the Mediterranean coast of Turkey, you will likely find April too cool (though, being from Seattle, we're okay with April at the beach!!) The interior of Turkey has a more continental climate, so if Cappadocia is on your itinerary, you will definitely want warmish clothing (especially if you plan on a dawn balloon ride) in April, but much less so in May. In fact, late May is usually downright hot (at least by Seattle standards). April is when lots of wild flowers are at their peak. There's nothing more romantic than wandering through ruins among blazing red poppies.
We were in Greece in late May, early June. The days were warm, short sleeves, and the nights and early mornings were quite cool. Our last night we were wrapped in sweats shivering while eating on the rooftop.
We just got home from Greece and Turkey June 9/07. We spent all of May in both places and we had really nice weather, just the odd day of rain. We went swimming and the water even in Northern Greece was warm. As for April, we were in Greece and Turkey in April a few years ago, it wasen't that warm to swim and we had several days of very wet weather. Our next holiday in Europe will again be in May, its a very nice month, the days are getting longer, its warmer and prices are still lower. Hope this helps you.