iPhone 6s & iPhone 6s Plus
At this point, it's important to know that the iOS 9 update will almost certainly be accompanied by brand new iPhone hardware. Rumors have outlined the iPhone 6s and iPhone 6s Plus in detail ahead of their launch and both look like exciting additions to the iPhone family of devices.
We're still not sure when they'll be out but it's safe to assume that we'll probably be looking at a two week gap between launch date and release date. At this point, our best guesses at the iPhone 6s release date are September 18th and September 25th.
With unique hardware rumored for the iPhone 6s and iPhone 6s Plus, there's a good chance they run unique versions of Apple's iOS 9 software.