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I can talk about a few of these things. I encourage others to contribute to this discussion as well.

TL;DR:
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!
 
@nordique, what a fantastic review of both!!! I did not know the A11 uses all cores. I assumed it operated like the A10 but added power and efficiency. Battery life has always been phenomenal on my i10. Battery life is the same or slightly better vs my 7+.

OP, depending on what your price range is and if you don’t care about the headphone jack, I actually recommend the 7. The stereo speaker on the 7 makes overall sound quality much improved over the 6s. If you don’t care for the stereo speaker and want something cheaper, I recommend the 6s. Like what was mentioned, the two chips are very similar.
 
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I’m on a 6s and I have the impression that it’s the first phone that is lasting years without feeling terribly dated in terms of speed. And iOS 12 will be a bit quicker. If Apple doesn’t implement some artificial slow down there no reason it should last several more years with reasonable speed.

absolutely agree on each point about the 6s:
1 great design that apple stayed with until the iPhone X. so not looking outdated even now.
2 the chip in the 6s is the A9. it was a huge jump from the A8.
3 from beta reports of iOS12, the 6s will also receive a speed bump due to greater optimization
 
I’m on a 6S now and opted for it over the 7 back in September 2016. I wasn’t disappointed and have found in real world use the performance difference is minimal. It’s made me realise I don’t need the latest iPhone to get the best experience and next month i’ll pick up an 8 on a two year contract.

The 6S is probably the best iPhone I’ve owned and has been solid.
 
I have the 6S and had the 7 twice. The 7 is faster on paper and has better graphics, you can notice it now and then but the same can be said if the iPhone 8.

The camera is a bit better, it has OIS which helps it.
The screen is a little better as it changed the colour gamut standard it uses.
Then you have stereo speakers that sound good and the water resistance with the solid home button.

Overall it’s a nice little upgrade on the 6S.

Weather the user will notice that I can’t say.
 
I’m on a 6s and I have the impression that it’s the first phone that is lasting years without feeling terribly dated in terms of speed. And iOS 12 will be a bit quicker. If Apple doesn’t implement some artificial slow down there no reason it should last several more years with reasonable speed.
I'm still using an iPhone 6 I bought in January 2015 and it overall still works fine, especially after I replaced the battery thru apple's program few months ago. with ios 12 no doubt I should be able to use for another year at least... and yet ofc I find myself really wanting to get a new X... lol
 
One Other differentiation between the iPhone 6 and 6S, was I noticed that the 6s had a much more bluish/starker tone the display and appeared brighter to me, but those are just my own observations.
 
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I would opt for the 7 just for the haptic button. Feels much quicker and nicer to use, and less chance of it going bad compared to the physical button on 6S. After using it on my 7, using the physical one on my wife's 6 seems almost a chore. I even held out for a long time for the haptic to come on iPad pros (don't care for the FaceID), but no luck so far.
 
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With the big write up earlier I can’t believe someone forgot to mention the 7 plus has more RAM than the 6s, 3GB vs 2GB.
 
One Other differentiation between the iPhone 6 and 6S, was I noticed that the 6s had a much more bluish/starker tone the display and appeared brighter to me, but those are just my own observations.

Plus the much improved chip and the RAM. That is why I like the iPhone SE over the 5s by a long shot. I bought mine for $50 and is used as a backup as well as an iPod for work. The A9 with the 2 GB of RAM makes that phone a beast, as well as the 6s.
 
There’s one thing the 7 has that the 6S doesn’t - Jet Black finish. Surely worth the cost for the best looking iPhone of that design type?
 
There’s one thing the 7 has that the 6S doesn’t - Jet Black finish. Surely worth the cost for the best looking iPhone of that design type?

Actually 2 things Jet Black and Matte Black. Some people black makes the biggest difference. The 7 has 2 true blacks where as the 6s does not. I personally prefer the 7 over the 6s in every level. The 6s is still a good phone, but as far as upgrades go I feel like the 7 is ahead of the 6s.
 
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Actually 2 things Jet Black and Matte Black. Some people black makes the biggest difference. The 7 has 2 true blacks where as the 6s does not. I personally prefer the 7 over the 6s in every level. The 6s is still a good phone, but as far as upgrades go I feel like the 7 is ahead of the 6s.
Annnd.. that’s a scienctific given. Lol
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If the lack of a headphone jack isn't an issue (for some it is) the 7 would be the way to go. IMO anyway, there are some nice improvements over the 6.
Side note. We both are macrumors 6502 lol
 
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