Just to add another: I would hold off unless you have a good reason to upgrade. I have the same iPhone 6 64GB and upgraded to iOS 11 because I wanted watchOS 4 on my Apple Watch.
My iPhone 6 was already a bit sluggish on iOS 10. It could take the camera 5 seconds or more to be ready to take a photo, the phone would sometimes freeze for a second when switching apps or launching apps (especially those that use the camera. Battery life was already not the best, but I could usually make it through the day unless I used the phone a lot. I usually top off the charge while I'm at work.
iOS 11 seems to have exacerbated the existing issues a bit, but it could be my imagination. It seems like the phone freezes for a second more frequently, and some apps take longer to refresh than before. It's most frustrating when you are in a situation when you need your iPhone to work quickly: Trying to take a photo, a quick calendar or weather check, send a quick email, etc. I can check my calendar or the weather much faster on my Apple Watch, but i open the calendar app when I want to view more calendar details and entries into the future.
I will be upgrading to the X (or to the 8+, depending on X reviews). I've been an iPhone user since the 3G, and in the past I've had old iPhones perform better with new operating systems. It was the new/updated apps designed for faster hardware that would make my previous iPhones feel slow. This time it seems to be the OS.
By contrast, my Series 0 Apple Watch Sport runs watch OS 4 just fine. Batter performance is even improved over watchOS 3.
Sean