As I conjectured back when it was first rumored that the new iPhone 6s would get 4K video recording capabilities, iPhone 6s owners will have a hard time maintaining space if they shoot even a moderate amount of Ultra High-Definition video on a 16GB iPhone 6s.
First and foremost, the 16GB iPhone features about 12 GBs or so of usable storage space. 4K video, while amazing looking, uses 375 MB of storage for every minute of video shot. It doesn't take a mathematician to quickly discern that 4K video will eat through 12GBs of storage space in relatively short order.