Two Devices, Same Day
This might be obvious for Apple enthusiasts but it's not apparent for those who don't follow iPhone news and rumors throughout the year.
Apple always releases its iPhones on the same day. The iPhone 5s and iPhone 5c emerged together and the iPhone 6 and iPhone 6 Plus shared a release date. iPhone 6s release date rumors haven't pointed to a split release which means that September 18th should serve as a joint release date for the iPhone 6s and iPhone 6s Plus.
That also means that the pre-order date should serve as the start of the early ordering period for both the iPhone 6s and iPhone 6s Plus.