Greece vs Croatia

We've done both .. and Turkey. 

I found the sailing in Croatia (amongst the Kornati Islands) fantastic but was a bit peed with the infrastructure (expensive marinas, tavernas that charged mooring even when you'd bought a meal for a family of five) - plus we used to go in May and it was a bit chilly still. 

Greece has lots of lovely places to stop, most tavernas will let you moor up for free if you eat there and it's nice and warm from mid May.  The Ionian is truly beautiful, but also very FULL.  Some places, like Fiscardo and Kioni, you need to be in harbour by 3pm to get a space (even in May).  The Dodecanese was much quieter but also much windier with some long (not quite line of sight) passages between islands (Tilos, Nisiros, Rhodes and Koss) we moored in some places without seeing a single other boat - though you can also come across an entire Sunsail flotilla of 12 matching boats ....

Turkey is similar to Greece (though windier still and more so as summer presses on), North out of Bodrum is a bit dull and the Gulf of Gukova just South of Bodrum isn't too exciting either - though it is quiet.  We had most fun sailing south to Knidos and then along to Datca and Orhaniye. Round the tip of the next isthmus to Loryma was nice and I'd like to carry on to Fethiye and Marmaris one day.  

Got to do them all, but Turkey was my favourite over all.

If you want to avoid seeing too many other yachts then I can personally recommed the Cyclades - By that I'm not talking about Mykonos / Santorini, but the north Cyclades.  We spent a nice week sailing around just Kithos, Serifos and Siros and Kea.  We admittedly didn't go far each day, but we still ended up in a different tavernas each night, with plenty in in sheltered bays and small marinas (that are cheap).  You also get the pleasure of the astounding Kithnos Sand Bar to take pictures of and make all your friends back home extremeley jealous.

In Croatia, for peace and quiet head to the Kornati (accessible from Biograd na Moru / Zadar area) - Split and Dubrovnik can tend to get a little overheated wth day trips and other sail tourists, but in the Kornati you can literally go for a full day without seeing anyone else.

I'd avoid August if at all possible - too hot, too crowded, and you have to be heading off the water pretty early if you want a spot in the marina - but if it's the only time you can do it in, I'd still recommend it! :)