There is a huge gap internally in so many ways, not just the processing power or the RAM but feature wise between the 6 and 6s, that its not worth it in any way to get a 6 in 2018
6s is still a fantastic device, capable of pretty much anything the latest iPhones can do. I have one running the 11.3 beta and its buttery smooth and fluid. It supports AR, has a powerful CPU and GPU (2.5 years after it came out!), excellent camera, upgraded bluetooth and cellular radios, band support, the same 2GB of RAM in the iPhone 8, the same 2nd touch ID in the iPhone 8, the same 7000 series aluminum as the iPhone 7. Like its really in the same league as the 7 and 8 as a purchase, whereas the 6 is not.
The 6 is just too old and too far behind in the tech specs these days. If you still had a 6 since launch and were trying to get another year of life out of it, that's a different story...iOS 11 runs well enough and the support is there so why not hold on.
But if you're shopping for a new phone to get in early 2018, stay away from the 6 - don't waste your money. SE or 6s is as far back as you should go, and there are incredible deals that pop up for the 6s (I am in Canada as well...keep an eye out for 6s or 7 deals)
Oh yah touch ID is also way improved. I noticed it when I use my GF touch ID on her 6.