"Hey Siri" support and possibly wireless charging case alongside AirPower charging mat.
4K Video from iPhone 6S Plus Shows Striking Advantage of Optical Image Stabilization
While the iPhone 6s doesn't have optical image stabilization, it does have digital image stabilization, which is the camera's software accounting for any shakiness. Giga Tech notes that digital image stabilization works well when the iPhone 6s is shooting in Full HD 1920x1080, but that it's not as effective when shooting in 4K, as shown in the video.
The video, which takes place in an old, abandoned airplane, offers a striking comparison of the two device's video capabilities. While the iPhone 6s footage looks fine on its own, the contrast to the smoother iPhone 6s Plus footage makes its shakiness seem more pronounced.