Optical Image Stabilization (OIS) for video is hands down the most significant improvement in the iPhone 6s Plus camera upgrade. The 6 Plus has OIS for stills (mainly helpful with low-light photos), but it wasn’t active when shooting a video or time-lapse. Now, with the 6s Plus only (not the 6s), we have active stabilization of all video and time-lapse content and the improvement is dramatic.
The time-lapse piece at the top utilized the Optical Image Stabilization significantly. The ability to create stable time-lapses anywhere, anytime, opens up so many doors. I can't wait to see what everyone creates with this.
If you had any doubts about going with the iPhone 6s Plus, they should all be absolved with Austin Mann's thorough review.