Regarding the weather - don't think its much different as its only about 3 hours away. In September the evenings will still be pretty hot.
The area is beautiful - Turunc is quieter than Olu and less commercialised, but you are in easy reach of Icmeler and Marmaris by boat or dolmus if you want a lively night out.
The nightlife in Turunc, is very low key, there's plenty of restuarants for a 2 week holiday, and there is a small amount of entertainment in a couple of bars at peak times, but the music stops by about midnight. If you just want a quiet evening meal or drink at night you will get that, without having to listen to loud music from the next bar.
Something that guests of ours noticed when they stayed there compared to the Olu area, is that they felt the restaurant and bar staff had more time for their guests, as they are not so busy.
It is very much a family-orientated resort, and in September seems to be mostly middle-aged couples.