iPhone 6S looks absolutely like iPhone 6. Difference is in letter S an the phone back.
And color of camera lens. It seems like kind of optical filter added to 6S.
Touch ID 2. Sensor looks outside exactly like previous model, but work about twice faster.
Photo have better resolution (12MP). Video have better resolution (4K). SLO-MO have the same 240 fps at 720p.
New feature Live photo is quite interesting. In addition to photo iPhone store couple of seconds of video. Video is up to 15 fps, with 1440×1080 resolution. Can be imported on PC as a .mov file with same name as photo. Video MPEG4 AVC/1440×1080@Variable(upto15fps) Audio PCM/44/16/mono.
3D Touch is also nice. It is actually an another dimension of the user interface. Interface of 6S now react on how hard you push. It may be simple context menu or preview/open depend on force of a touch.