Update: if you feel the need to see the two phones working against one another in a lovely moving set of pictures, then we've got a treat for you with our new, in-depth test of two of Apple's newer handsets.
We tested battery run down, camera prowess and speed, so you can decide once and for all which is for you below:
With a much-improved design and the necessary upgrade to the battery, the iPhone 6 was one of the phones of 2014. Apple needed a big hit, and it got it.
However there was always the worry about 2015's model - whispers that Apple might skip the 'S' model and go straight for the iPhone 7 were sadly unfounded and we got the predictable slight upgrade with the iPhone 6S.
But Apple tried to convince us differently, using the tagline: 'The only thing that's changed is everything'.
Well, except the battery life, design, interface - but there were big improvements to the camera and additional features like 3D Touch and Live Photos that aren't available anywhere other than on the newer 6S models - so are they enough of an upgrade?
Speed and performance
One of the big changes Apple makes every year is to add another number to its processor, and this year the A9, unsurprisingly, whips the butt of last year's A8. We're talking 70% faster in general use, and 90% more powerful when it comes to graphical prowess.
But we're getting to the point now where such upgrades are slightly irrelevant, right? Our phones are now more powerful than our computers were a few years ago, and they're supposed to last well over a day on a sliver of battery.
As you can see in the video at the top of the page, the iPhone 6S annihilates the 6 for general app opening and closing, and if you think how often you do that daily, those seconds will really start to mount up.
Let's take a gander at the benchmarks: the iPhone 6S scored over 4400 on the GeekBench 3 test, where the iPhone 6 could only muster a paltry 2902.
That's more powerful than the iPad Air 2 and closing in on the Galaxy S6 clan - known as the most powerful smartphones on the market.
Make no mistake: the iPhone 6S is a hugely powerful phone that's capable of running some impressively graphically intensive games. If you want to future proof yourself for the next two years, go for the newer model.
Aesthetically, you're not going to miss much by sticking with the iPhone 6. That's not to say the iPhone 6S isn't a good looking phone - it is, in fact it's gorgeous - but it also looks almost identical to iPhone 6.
They both have a slim metal unibody with curved edges, but there is a slight difference in dimensions and weight, with the iPhone 6S coming in at 138.3 x 67.1 x 7.1mm and 143g, while the iPhone 6 is slightly slimmer and lighter at 138.1 x 67 x 6.9mm and 129g.
The iPhone 6S is also stronger, as it uses a 7000 Series aluminium alloy, which should help avoid a repeat of BendGate.
Both phones come in space grey and silver, but Apple has added gold and rose gold options into the mix with the iPhone 6S, so if one of those is your shade of choice you're out of luck with last year's model.
You'll really struggle to feel the difference in the hand, but if you're comforted by the extra strength, then the iPhone 6S is your friend. But if you want the lightest and thinnest model, the older model will suit your needs nicely.
Both the iPhone 6 and the iPhone 6S have 4.7-inch 1334 x 750 screens, but Apple claims it's used the strongest glass in the industry for the iPhone 6S. We still wouldn't drop it though.
Quite honestly, 3D Touch is probably the number one reason you should be thinking of picking up the iPhone 6S. The new method of interacting with your phone could be as pivotal as the first time you used multi-touch, and we're inclined to agree.
It's worth noting that at this point it only works with native Apple apps - Mail, Messages, Safari etc. - and often will do as much as a long press would. The ability to 'pop' into the app is the differentiator, as when you get used to it you'll find that you save oodles of time.
Beyond that though, the two displays are identical. Both could still do with a touch more effort in the deepness of the blacks, and the sharpness of the screen is falling woefully behind the competition.
They look great though if you don't peer too closely - and the extra strong glass Apple's cooked up doesn't seem to have had an effect on sensitivity or the brightness.
Want to see how 3D Touch works? Check out our video just below for a full look at the new screen tech.
Let's kick off with the rear-facing camera. You'd expect that with 12MP on offer pictures would appear sharp, clean and full of more color.
However, there's an issue that all camera manufacturers come up against: packing in more pixels makes it harder to capture bright and clean photos.
HTC tried to go the other way and use a 4MP sensor (dubbed UltraPixel) with the aim of capturing better, brighter and cleaner pictures, but it couldn't convince the public that fewer megapixels were a good thing.
It feels a bit like Apple's bowed to the pressure to bring a higher number sensor to the 6S, and the odd thing is it really doesn't perform any differently to the 8MP one on the iPhone 6.
Odd for a couple of reasons: firstly, why bother if the results are going to be virtually similar? You can see in the following samples that there's very little difference between the two.
Secondly, it's impressive that Apple has actually managed to improve low light performance in the 12MP sensor on the iPhone 6S. If you look at the rabbit picture, you can see the new sensor is slightly ahead in information absorbed, and that's something we didn't expect.
The front facing 'selfie' camera is just simply a cut above. The Retina flash, which boosts the screen to three times its normal brightness to illuminate your face, works pretty well and gives you options for photos where there were none before.
The extra sharpness is also much, much better - in short, if you're a selfie fiend, there's no question.
Here's a quick look at the iPhone 6S camera and its 4K video capabilities.
Now here's an odd thing: the iPhone 6S has worse battery life than last year's iPhone 6. It makes sense when you drill into it: the old phone had a 1810 mAh battery pack, and the new iPhone only comes with a 1715mAh option.
There are multiple reasons for the smaller battery pack: a larger motor needed for the 3D Touch interface, and the extra space needed for the pressure sensor.
But the bad news is that this doesn't get offset by efficiency improvements in the upgraded iOS 9 - the iPhone 6S performs worse in our battery tests.
For the Full HD 90 minute video test (at full brightness) the new iPhone dropped 30%, compared to only 26% for the older model.
Given battery life was already a concern for iPhone users, bringing out a new phone with a shorter time between charges is criminal.
Price and availability
The iPhone 6 has been out for a year, so it's readily available from numerous stores including Apple's own shops and website, where it's dropped to £459, US$549 or AU$929.
The iPhone 6S has been out for a few months now, but costs quite a bit more than the iPhone 6. The iPhone 6S starts at £539, US$649 or AU$1,079. That isn't likely to drop down until we see the iPhone 7 release either.
If you're stuck on the iPhone 5 or 5S, then you've got a conundrum here. The iPhone 6S has loads of great upgrades, with 3D Touch easily being the pick of the bunch. Its 12MP camera does offer a slight jump forward, and you'll at least get half an hour's joy out of Live Photos.
However, there's the issue of price. The new phone is a lot more expensive, and with a similar camera, same chassis and a lower battery life could make you think twice about the older iPhone 6 - or perhaps wait another year for the iPhone 7.
The iPhone 6S Plus - especially if you're after a better battery life - might well be worth a look if price is less of a worry for you, but either way you've got a bit of a conundrum if you're ready to upgrade your iPhone.