I think the upgrade would be more substantial from your 6s to the iPhone 8 versus the 7.
Especially being you don't need a lot of storage, and the iPhone 8 offers the improved stereo speakers, wireless charging, water resistance, dual camera and it significantly improved over the iPhone 7.
However there are some heavily discounted prices on the iPhone 7 128 GB if you find one.
Just made the jump from a 6s 64gb to an 8 64gb myself. Had the 6s for 2 years exactly just didn't want to deal with the battery issues anymore and my phone was just kinda beat up, so went ahead and made the purchase. In the 2 years of use on the 6s, I've got about 34gb of space left. That's with a good amount of camera and app use. 8 64gb suits me just fine as well since I like to use my new phones from scratch rather than load previous setups from iCloud (still a great option, just preference).
Btw, the jump from the 6s to the 8, was well worth it. Performance for days. Excellent smartphone. Does everything I need it to and then some with the new A11 processor. Fantastic new camera only bested, just slightly, by the 8+'s camera. Just see it as the more refined version of the 7. Other than making a very legit argument at how a company the likes of Apple, couldn't be bothered to make a salient design overhaul on a more than 3 yr old device *like really, Apple?! Cmon*, can't see how you'd go wrong with the 8+ purchase.
7 plus it is (but there is no headphone jack)
8 plus shouldn't even exist!
In this day and age where people share photos mainly via social media (and consumed via mobile devices), the stated camera improvements are just academic...
Quite honestly 6s plus is not bad at all.