As both give similar battery life the main differences are pretty much:
6s plus:
1080p screen
headphone port
"old" home button
7:
750p screen
no headphone port
"new" home button
stereo speakers
water resistance IP67 rating (that Apple does not warranty, but to my knowledge other companies that offer water resistance do not either, so it's more of a peace of mind type protection)
It's a tough choice because on specs alone the 7 seems to offer more. But the screen is what is used the most, so having the better screen when you're looking at it is definitely nice. I don't think long-term support is an issue with either device as Apple has a good track record of supporting older devices, plus a lot of people upgrade before Apple drops support for the device. Both offer 3D touch, Touch ID and Apple Pay.
My choice would be the 6s plus for sure. (I've owned both the 6s plus and the 7 so I'm comparing in my mind which I'd prefer from a daily usage point of view)