There are these following differences that one can make out between the two iPhone models, iPhone 6 and 7:
Size and Design
Apple has incorporated a 3.5mm Lightning connector in the iPhone 7 box and apart from this there is no such major difference in the size except in their weight.
The size and dimension of iPhone 7 is 138.3 x 67.1 x 7.1 mm weighing 138 g (4.87 oz)
The size and dimension of iPhone 6S is 138.3 x 67.1 x 7.1 mm weighing 143 g (5.04 oz)
Where the iPhone 7 has enhanced in its solidness with a Series 7000 aluminum from the iPhone 6S model which has a IP67 tidy and water resistant body that allows the telephone to survive even when completely submerged in the water for up to 30 minutes.
Minor Changes in Display
There are some minor changes in the display between both the phones and here they go:
The iPhone 7 display is of 4.7-inch having a LED-backlit IPS LCD, with 1334 x 750 pixels (326 ppi) having 65.6% screen-to-body ratio
The iPhone 6 display is of 4.7-inch having a LED-backlit IPS LCD, with 1334 x 750 pixels (326 ppi) having 65.6% screen-to-body ratio
Apple has boosted up the peak brightness on their iPhone 7 by 25%, by introducing a wider (P3) colour gamut and with better support for colour management along with a "cinema standard" display.
The iPhone 7 has in it the Apple A10 Fusion chipset with a Quad Core CPU, a Six Core GPU, and RAM of 2GB
whereas the iPhone 6S is having an Apple A9 chipset, with the Dual-core 1.84 GHz Twister CPU, a PowerVR GT7600 GPU with 2GB of RAM.
The iPhone 7 is sporting a 12 megapixel sensor sensor camera in the Rear that has f/1.8 aperture, with Focus Pixels and Optical Image Stabilisation It is also having a quad-LED (dual tone) flash with 4K video recording feature. The Front Camera is of 7MP sensor with f/2.2 aperture and 1080p recording.
On the other hand the iPhone 6S has the same 12 megapixel sensor in its Rear with f2.2 aperture along with Focus Pixels, and Software Image Stabilisation. It too has dual-LED flash and the capability of 4K video recording. The Front Camera is of 5MP with f2.2 aperture and 720p video recording.