I can talk about a few of these things. I encourage others to contribute to this discussion as well.
The difference in performance is negligible. They are very similar in performance. Both very similar phones.
More in depth:
First, the The A9 CPU is a very powerful processor still, as is the A10 ... the A10 has multiple upgrades over the A9, but the high performance cores of the A10 and the A9 are very similar. Most of the speed difference, as minor as it is, comes from the solid state home button (more below) and on the higher clock speed. But in every day use, they are almost identical. The 6s is truly remarkable how well is has held up, and the A9 is one of the best designed chips in a smartphone or tablet.
(Fun fact: unlike the A11 which actually uses all the 6 cores congruently, the A10 only uses 2 of the 4 at a time...either the low power or either the high power. Never all 4 at once. The A10 is still plenty powerful and the bugs have been sorted out, but in the early days of iOS 10, the iPhone 6s performed slightly better than the 7 and at the time many people postulated this was due to a bug involving the high & low performance core tasks. This is not the case anymore, and hasn't been since like iOS 10.1, but it's something that is sometimes brought up when discussing the two in terms of performance)
In terms of GPU, the iPhone 7 has a minor GPU update to the iPhone 6s, it's the "next" version of the one in the 6s but it was more of a half-step upgrade. For me the remarkable thing was how much more efficient it was than the monster GPU in the A9.
In terms of RAM, identical between the two. Same type, same frequency.
Speed feels better on the 7 due to the lack of a physical button so home button presses are detected much quicker (via software) as opposed to requiring the physical press. So that leads to the perception of improved fluidity, as one can actually access the home screen or app switcher quick *if* using the buttons - even though both are actually identical in the OS speed as of iOS 12 beta 10 (it's identical if using the 3D touch app switcher).
In terms of the horsepower improvements, most of the speed improvements comes from the fact the A10 high performance cores are clocked higher - it can run at 2.34GHz vs the 1.85GHz in the A9...that's quite a bit higher clock.
The 7 has nicer colours on the screen, because of the wider P3 gamut. The 6s has a more blue hue to the display though, I know some people who prefer that sRGB gamut, but I like the wider colour gamut personally, the colours pop out a bit more for me, but its a minor difference in day to day use. Resolution, display size etc is identical. Brightness slightly better on the 7.
It has true IP67 dust & water resistance, but the 6s has some minor water resistance with gaskets outlining the internals, as well.
The cameras are very similar.
For pictures: Wider aperture on the 7. I have used both extensively, OIS is nice on the 7 and helps with low light photos but one would be hard pressed to note the difference for every day pictures. If zooming in on two pictures on a 5k display is your thing, you'll spot differences but if you're sending them to people or viewing them on a 1080p TV or something, or on your laptop, you won't be able to tell the difference. Video capability is identical, OIS aside. It's a very minor update. The front facing camera looks nicer though, more detail vs the 6s but both the 7 & 6s are significantly (*significantly*) better than the iPhone 6 in that department.
For video: In terms of video performance, the 7 camera output is more or less identical to the 6s Plus camera. Same features, same focal length. To this day, both phones have fantastic video cameras...but the biggest difference between the non-plus 6s and 7 here is the OIS. It can make video seem "more professional" and it can certainly help in quality.
Battery life is a bit better on the 7, but not significantly so. A slightly larger capacity, but most of the improvement in battery comes from how much more efficient the A10 is (thanks to the low power cores and more efficient parts). 6s has the headphone jack which some people value a lot.
Both have 3D touch, live photos, 4k, same slow mo, both have raise to wake, both have handless hey-siri, there's a bit more haptic feedback with the 7 due to the second gen engine, the 7 also has stereo speakers which are nice. There is more LTE band support in the 7, but they both support LTE-A, both support same Wifi with MIMO (which most people don't even use), they both support same Bluetooth 4.2, both support same Geolocation, both can do VoLTE but that's a standard feature now.
Both have the same second gen touch ID. Both more or less look the same lol.
Both the A9 and the A10 will last for years, they are still very powerful chips. Both of them will be very fast for years to come.
They are both very similar phones, I wouldn't recommend one over the other today...but that is because new deices are around the corner. I think the 6s has more value, because it can be found cheaper yet it performs similarly.
If you want to be generous to your friend though, go for it!