The main and the biggestdifference between these two models is their size.
iPhone 6s is about 4.7 inches and have a resolution of about 1920x1080.
iPhone 7 is about 5 inches big and has a high class resolution of about 2560x1440.
Here below is the explanation of what i am saying in a picture.
Back design.Both of iPhone 6s and iPhone 7 has a very fine design specially both of their back are very different and unique. Let me explain you what i am saying.
Headphone Jack:Apple is changing its iPhone 7 headphone jack form stereo to lightning jack. This will bring very changes in the quality of the sound. It will also reduces the consumption of space and will also help in the making of slimmer design.
This is a picture of a lightning and a regular jack.
Micro USB Port and Lightning Cable:
Every one is aware that apple has its very own jack. But we have noticed that apple is changing its all ports with the new latest lightning jack. This jack has its own two advantages one is that this jack is faster and this jack reduces the chance of damaging the port and it also gives a advantage of two sided input means you can either attach this port in upside down or down side up it don't matters no more here is a preview of both the cables down bellow.