I think the reason for vertical videos is (partly) Apple. Look at their website. ALL pictures of the iPhone camera (video + photo functions) show it vertically. Look at their promo videos. ALL people shooting videos are holding it vertically. Maybe somebody should tell Apple about that.
Other than that: iPhone 6s (and iPhone 6s Plus) shoots 4K videos ONLY with 30 frames per second (and interlaced, not progressive; I'd call that "4K-ready"). You need additional equipment (new TV set!) for being able to watch your 4K videos in full resolution. You have to sit way too close in front of a monitor for being able to see a difference between HD and UHD (=4K) in moving pictures. The NEW Apple TV does NOT support 4K video! (I still have no clue how to bring the iPhone 6s's 4K on a tv set… HDMI cable?)
Even the OLD Apple TV (and your maybe even more than five year old tv set) supports 1080p60 videos (iPhone 6, iPhone 6 Plus; and as an option of the new iPhones) easily. Those sixty frames per second progressive (I'd call that "full HD+") are much more important than that 4K crap in my honest opinion. For me personally the only benefit of the iPhone 6s Plus could be the optical image stabilization which works even on videos (iPhone 6 Plus: optical stabilization only for photos) but I'll have to try that out first for being able to judge.
But the digital image stabilization of iPhone 6 and iPhone 6 Plus works great already for videos (try that out please, you have to activate 60 fps in the iOS settings; it's 30 fps per default and it switches back to 30 fps when you update to iOS 9). I doubt that the iPhone 6s supports digital image stabilization in 4K videos.