If you want to record the smoothest 4K video with an iPhone, skip the 6s and go with the 6s Plus.
The two new iPhones appear to be identical in almost every way except screen size, but in a new 4K video comparing the digital stabilization of the iPhone 6s against the optical/digital stabilization on the iPhone 6s Plus, the new camera on Apple’s bigger-than-big iPhone is clearly superior.
Check it out for yourself: