Please try to make sure you are not missing the spectacular sunsets from the coastline. Discuss with experienced yachtsmen and friends for some real advices. Go for best price high quality service.
I chartered a Jeanneau 35 from Skiathos in late September 2010 and had a great time. We used Athenian Yachting and were very happy with the yacht and the handover. I believe it can get very crowded there in August, like the Ionian, but it was fine when we were there.
Did the Saronic SH flotilla in the past and loved it. The only "flotilla" aspect is that you go to a certain place each day. Beyond that you are free to set off when you want and do whatever you want each day. For a holiday I would happily choose this sort of loose flotilla as the no hassle aspect makes for a much better experience.
FYI most of the skippers in our group were very experienced and at least half had their own boats in the UK as well. Do not think of a flotilla as being only for the weak and timid. Its all about the fun.