I want to go for a beach holiday in Kusadasi, Turkey this summer with a family but can't decide -- should we go late May or better early September?
What are pros/cons going late May/early September?
The answer will depend of course on what you're looking for in a vacation. But September looks better since the water and air would be warmer and there will be less rain.
The weather is decent in May and September, with September being a bit warmer. May is really off-season in that region. In September the place is less crowded than during July and August, but you may still feel the spurs of the high-season.
To conclude, both are fine. Check the prices and pick the period which suits you best.