Released in september 2015 Iphone 6s is an upgrade to the September 2014 released iphone 6 but I personally don’t find it worth upgrading because of following reasons : -
The Battery backup is overall decreased in digits as well as in usage
It includes a 3D touch which is not usefull at all.
The camera is now 13Mp but the qualities that came out gives you the same feeling.
and there are loads of reason
WHY YOU SHOULD NOT UPGRADE