I really didn't need to upgrade my 6s, the battery life was still solid--it had been replaced under the Apple Battery Replacement Program, iOS 10.3.3 was stable and the phone was in great shape. A family member was in need of an upgrade and I'm always tempted by technology so I bit the bullet. I went from 6s to 8+. Better camera, incredible battery life, improved screen, more real estate on the screen, etc. My 6s was paid off so I could have saved a few $ on the monthly bill, but I traded in a 5s (from family member and gave them my 6s), received a $100 credit for it and even with the 256 8+ my monthly payment is actually $.01 cheaper. Personally speaking, I much enjoy the design and find the latest version to be very refined with the addition of glass back.
I agree though, take your time. Think it through and get to a store where you can get your hands on both 8 and 8+ to see if the upgrade is worthwhile to you. It is clear, many of us who've upgraded are happy. Many are happy with what they have, and some are waiting for the X. Your decision is the only one that matters.